What was the revenue for ASX's testing business in the fourth quarter of 2020
 think that, we are looking at a huge growth rate ahead of us. And I can't really give you a number.
But over the last few years, we have been faithfully collaborating with a lot of our key customers, and trying to come up with design applications that can improve the efficiency, leverage the success of the HPC bigger brain, and also the success of very powerful network in cloud with the 5G data rate and low latency. I think we're seeing more application in all areas, but all of the little devices for the mass market, that is really the sweet spot that ASE has been designing for. And with our key customers, with our early success, I think ASE today almost become the first choice. For example, in 5G, in most of the unique SiP, ASE has always been the first one to engage with our key customers. And I think that trend, hopefully will repeat in 2021 that we will have an even higher confidence, and maybe we'll be able to give you a projected market size, based on the effort.
But in the last few years, we couldn't give you that number because we're primarily working with few customers. But now we're seeing a broader, more diversified portfolio. And we gradually understand the design rule, the value, the physical, electrical, the cost performance. So we're just building the database to that. Our fully automated factory really was part of the overall architecture to accommodate that because if we don't understand how the sensor interact physically and electrically with one another, it's very, very difficult to build high multiple die, high level integration with overriding sensor using different materials, different configuration. And I think that is something have become more obvious to us through the effort of the last few years. It's a long answer, but I think it's a key answer -- is a key message I would like to deliver to all of our partners investors.
Gokul Hariharan -- JPMorgan -- Analyst
Got it. Thank you very much, that's very helpful. Thanks.
Operator
Now the line is open to Roland Shu, from Citigroup.
Roland Shu -- Citigroup -- Analyst
Hi, good afternoon. I think the first question is still for the gross margin. So, look at your IC ATM gross margin in fourth quarter, it improved by 2.4 percentage points with the overall IC ATM revenue was slightly better than 3Q of last year. However, I look at this testing revenue, actually it's been declined a lot in 4Q. So, but still, your gross margins improved a lot. So, Joseph just said, you have a lot of these efficiency improvement and some also with a better pricing environment. Question is, is this improvement on gross margins are write-off or are we expecting the gross margin improvement for this IC ATM will continue even though our testing business declined in 4Q?
Joseph Tung -- Chief Financial Officer
With the higher loading that's continuing with very strong business momentum and the continuous effort that we're putting in terms of improving our overall efficiency, and also with the closer collaboration with SPIL to create further synergy, I do believe that in '20 -- this year, we'll continue to see margin improvement at the gross level.
Roland Shu -- Citigroup -- Analyst
Okay, Yeah.
Joseph Tung -- Chief Financial Officer
Although there are some headwinds in front of us, including the strong NT dollar and also, as you mentioned, the -- we're still trying to -- we're in the process of recuperating, some of the lost test business, which tend to have a higher gross margin. But all -- putting all of these together, I think that there are some plus and minuses. But we still, we're still fairly confident that we will continue to see margin improvement going forward.
Roland Shu -- Citigroup -- Analyst
Understood. Yeah, and for your testing business, actually, in past couple years you had a big amount of the testers and then you also would like to increase this testing business. However, I think in 4Q, the revenue for the testing was the lowest quarter in last year. It was even lower, like 4Q last -- 2019. We think, the EAR probably was the reason for this lower testing revenue,
 think that, we are looking at a huge growth rate ahead of us. And I can't really give you a number.
But over the last few years, we have been faithfully collaborating with a lot of our key customers, and trying to come up with design applications that can improve the efficiency, leverage the success of the HPC bigger brain, and also the success of very powerful network in cloud with the 5G data rate and low latency. I think we're seeing more application in all areas, but all of the little devices for the mass market, that is really the sweet spot that ASE has been designing for. And with our key customers, with our early success, I think ASE today almost become the first choice. For example, in 5G, in most of the unique SiP, ASE has always been the first one to engage with our key customers. And I think that trend, hopefully will repeat in 2021 that we will have an even higher confidence, and maybe we'll be able to give you a projected market size, based on the effort.
But in the last few years, we couldn't give you that number because we're primarily working with few customers. But now we're seeing a broader, more diversified portfolio. And we gradually understand the design rule, the value, the physical, electrical, the cost performance. So we're just building the database to that. Our fully automated factory really was part of the overall architecture to accommodate that because if we don't understand how the sensor interact physically and electrically with one another, it's very, very difficult to build high multiple die, high level integration with overriding sensor using different materials, different configuration. And I think that is something have become more obvious to us through the effort of the last few years. It's a long answer, but I think it's a key answer -- is a key message I would like to deliver to all of our partners investors.
Gokul Hariharan -- JPMorgan -- Analyst
Got it. Thank you very much, that's very helpful. Thanks.
Operator
Now the line is open to Roland Shu, from Citigroup.
Roland Shu -- Citigroup -- Analyst
 Hi, good afternoon. I think the first question is still for the gross margin. So, look at your IC ATM gross margin in fourth quarter, it improved by 2.4 percentage points with the overall IC ATM revenue was slightly better than 3Q of last year. However, I look at this testing revenue, actually it's been declined a lot in 4Q. So, but still, your gross margins improved a lot. So, Joseph just said, you have a lot of these efficiency improvement and some also with a better pricing environment. Question is, is this improvement on gross margins are write-off or are we expecting the gross margin improvement for this IC ATM will continue even though our testing business declined in 4Q?
Joseph Tung -- Chief Financial Officer
With the higher loading that's continuing with very strong business momentum and the continuous effort that we're putting in terms of improving our overall efficiency, and also with the closer collaboration with SPIL to create further synergy, I do believe that in '20 -- this year, we'll continue to see margin improvement at the gross level.
Roland Shu -- Citigroup -- Analyst
Okay, Yeah.
Joseph Tung -- Chief Financial Officer
Although there are some headwinds in front of us, including the strong NT dollar and also, as you mentioned, the -- we're still trying to -- we're in the process of recuperating, some of the lost test business, which tend to have a higher gross margin. But all -- putting all of these together, I think that there are some plus and minuses. But we still, we're still fairly confident that we will continue to see margin improvement going forward.
Roland Shu -- Citigroup -- Analyst
Understood. Yeah, and for your testing business, actually, in past couple years you had a big amount of the testers and then you also would like to increase this testing business. However, I think in 4Q, the revenue for the testing was the lowest quarter in last year. It was even lower, like 4Q last -- 2019. We think, the EAR probably was the reason for this lower testing revenue,