Abstract: In this paper the curves of longitudinal wave velocity in steel versus temperature from room temperature to 1200℃ are given The velocity variation with temperature on heating and cooling is discussed and the effect of the austenite transformation and interfaces of grains with coefficient of elasticity less than it in grains on particle displacement is explained as the reason to cause the hysteresis phenomenon here.At the same time,the discovery of the hysteresis is useful to investigate microcosmic process in austenite transf rmation through interreaction between sound and material crystal structure.Pointing out that the austenite transformation temperature can be deduced from the curves obtained,especially Arl and A 3.In addition,the results obtained are also significant guidance for precisely determining the flow position and avoiding the temperature region in which the velocity varies drastically in ultrasonic testing of hot steel blooms and slabs.