average cycling speed 12km/h I am now in training for cycling. Long time didn't got into bike. I am using a 21 gear mountain bike with highest gear, and around 70 - 80rpm. What's surprise me is my average speed only 12km/h(recorded from strava) and 14km/h(recorded from mapmyride).

I was wondering whether i am using the wrong gear? I thought that the higher with the same rpm will move faster than a lower gear with same rpm.

12km/h is much slower than my expectation. I expect 70rpm would be 20km/h or more.
You can calculate your speed vs cadence using this online calculator. You want your cadence to be roughly 80 rpm (60 being the lower bound). 

With a 42t chainring and 14t cog in the back (not uncommon for a 26" mtb -- this is big ring in front, small in back), you should be doing around 31 kph at 80 RPM, so chances are you're either spinning too fast in a too low gear or spinning too slow in a too high gear.