The blades of fan are effectively modified by increasing the degrees of inlet and outlet angle of the blades as well as adding blades. The flowfield is calculated in terms of 3-D theory. On the basis of the results the velocity distributions on the surface of three given blades are calculated.The distribution show that high efficiency range can be enlarged and the head of pressure can be increased considerably, and that the modified impellers have better aerodynamic characteristics and capability to do work.The results of the experiments are basically in agreement with tile theoretical analysis.