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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.