An ultrasonic vibration cavitation erosion experiment was performed to study the thermal effect during the erosion process. The ring affected zone was observed on the sample surface around the erosion pit at the incipient stage of the cavitation erosion. The results of the surface testing on roughness, hardness, and chemical composition proved that the zone was caused by thermal effect,and that the zone surface experienced a tempering process with the temperature higher than 300°C. Numerical simulation results show that the high temperature domain in the bubble directly contacting the solid wall is a necessary condition for the occurrence of the tempering process on the zone surface, or the heat in the bubble can hardly be transferred to the solid wall under the effects of the great temperature gradient in the bubble and the quick cooling process in the water.