Abstract: The nonlinear parameters of ferritic stainless steels with different grain sizes were measured by nonlinear ultrasonic testing technique, and the results were compared with that of the traditional ultrasonic technology. The results show that with the increase of the grain size of ferrite stainless steel, the variation of sound velocity is not obvious, its relative change is only 1.89% and the relative change of attenuation coefficient is 5.65%. The nonlinear parameters whereas decrease continuously with the increase of the grain size, which is due to a large part of the nonlinear effect being generated in the grain boundary when the acoustic waves propagate in the material. With the increase of grain size, the grain boundary area is reduced, so the nonlinear effect is weakened.