Abstract: P91 steel is an important bearing steel material in nuclear power. It is meaningful to make early damage evaluation for P91 steel. A nonlinear ultrasonic test is implemented to P91 steel specimens heat-treated at different temperatures based on the theory of nonlinear ultrasound. The square of the fundamental amplitude and the Second-harmonic amplitude have a good linear relationship as progressively increasing the excitation voltage. The nonlinear parameter is calculated by analyzing the spectrums of fundamental and second-harmonic. The result shows that the nonlinear parameter increases with heat treatment temperature. It can be used for early thermal damage evaluation of such materials.