Abstract: Cavitation erosion behavior of two kinds of martensitic stainless steel used for hydraulic turbine blades was investigated in distilled water and simulated Yangtze River water through an ultrasonic vibration system. The microstructure and the eroded surface morphology were observed by SEM and EM, and the results were in contrast with those of 13-4 stainless steel. The results showed that the microstructures of all the three kinds of stainless steels consist of martensitic matrix, ferrite and residual austenite. After refining with C, S and Si decreased and Ni, Mo and Mn increased, the stainless steel has an obviously higher resistance to cavitation erosion than that of original stainless steel and 13-4 stainless steel. The weight loss of the new type alloy due to stable cavitation erosion is less than 7 mg/h, i.e., about 75% of the original stainless steel and 25% of 13-4 stainless steel. During the cavitation erosion, the soft ferrite phase drops out form martensitic matrix first and then the hard martensite, and the surface morphology of fatigue source was thus observed clearly.