same decibel level the serial fan f 1 of the invention provides a higher air pressure and air volume than the conventional serial fan r at high or low rotational speeds except for the described first air guiding recesses 504 defined on the first edge 5031 the first air guiding recesses 504 can also be defined on the second edge 5032 of the first connecting element 503 or defined on both the first and second edges 5031 5032 of the first connecting element 503 additionally the air guiding recesses defined on the first edge 5031 of the first connecting element 503 can symmetrically correspond to the air guiding recesses defined on the second edge 5032 of the first connecting element 503 or the air guiding recesses defined on the first edge 5031 of the first connecting element 503 and the air guiding recesses defined on the second edge 5032 of the first connecting element 503 can be asymmetrically or alternatively arranged in other embodiments the first connecting elements 503 can be ribs or static blades in fig7 a serial fan f 2 of a second embodiment comprises the first fan 5 and a second fan 6 b the serial fan f 2 differs from the serial fan f 1 in that the second fan 6 b has the same structure as the first fan 5 the serial fan.