Abstract: The ultrasonic sound velocity of grain size of 304 stainless steel under different heat treatment was measured and the spectrum characteristics of the first echo, second echo, and signals between the first and the second echo were analyzed. It was showed that the larger the grain size, the higher was the speed of sound when ultrasound propagated in the stainless steel. The first, second echoes and the signal between the first and the second were in the frequency band. With the increase of grain size, frequency decreases and the role of scattering enhances.