Abstract: The control strategies have essential influences on the dynamic stability of modern wind turbines. Thispaper presents a theoretical investigation on this topic. The investigation is motivated by a vibration measurement of4 wind turbines with capacity of 1. 5 MW in a wind park. The wind turbines always get into a violent axial vibrationabove rated wind speed,which is finally confirmed as an instability event associated with pitch control system. Inthis paper the mechanisms of control strategies influencing the dynamic stability are revealed,and the stability en-hancement by means of time delay in pitch control is ascertained. The results show that the optimum control of rota-tional speed for maximizing energy capture below rated power will improve the dynamic stability suppressing the axi-al unstable vibrations of wind turbines. Above rated wind speed,two control modes can be used to keep the poweroutput equal to rated power. One is fixed speed and variable pitch,and another is variable speed and variablepitch. However the control mode of fixed speed and variable pitch can cause unstable axial vibrations of wind tur-bines, pitch control with variable speed can suppress the unstable vibrations. Additionally an appropriate time delayτ in pitch control can effectively enhance the dynamic stability of wind turbines. The optimum time delay is Topt (Tis the period of the axial vibration of wind turbines).