Abstract: Over-spheroidization of pearlite will cause the degradation of material mechanical properties, whereas the conventional linear ultrasonic testing is not sensitive to this microstructure change. To achieve different spheroidization states of pearlite, different heat treatment profiles were applied to T8 steel samples. The nonlinear coefficient β was measured correspondingly by the experimental system. Results showed that there was no notable change in ultrasonic velocity and attenuation coefficient with the increase of the spheroidization degree of pearlite. However, the nonlinear coefficient β showed a significant increase and it can be used to characterize the microstructure evolution of the material.