Abstract: 304 stainless steel samples in three different solid solution states after sensitization treatment at 650 ℃ for 2, 6 and 16 hours were evaluated employing ultrasonic testing in order to discuss the feasibility and ultrasonic nondestructive characterization parameters for the sensitization degree of 304 stainless steel. The results show that the sensitization degree of 304 stainless steel increased with prolonging the sensitization treatment time. Using attenuation coefficient, peak frequency and peak amplitude could nondestructively characterize the sensitization degree of 304 stainless steel, but ultrasonic velocity could not respond to microstructure changes under different sensitized conditions. With prolonging the sensitization treatment time, the attenuation coefficient increased gradually, which was mainly attributed to the deposition of plentiful chromium carbides along the grain boundaries, increasing the scattering of ultrasonic wave. The peak frequency exhibited a shifting to high frequency direction and the peak amplitude increased obviously with prolonging the sensitization treatment time.