Abstract: A Violent vibration emerged in the fan of a 50W wind-driven generator and caused manycracks at its surface as the velocity of wind reached about 8-10m/s. Some of the fans even broke down near the root(about J/3 length to root). Theoretical and experimental analysesof the faults show that the violent vibration is caused essentially by flutter due to flow -induced vibration. Therefore, the fan was modified into two new types (FD-1 and FD-2) to prevent the forced vibration and flutter. According to the flutter theory,the attack angle of the original fan was changed and its weight center was moved ahead so that the modified fans possessed enhanced natural frequence and decreased reduced velocity.The modified fan FD-1 was investigated by testing on the running car. The windmill generator with this fan started smoothly and ran very well. When the velocity of wind was about gm/3, its maximum power was about 74W,nearly 1.5 time of the original. No flutter phenomenon occured.