Cavitation damage in hydraulic machines is still unavoidable as the downsizing and the increase in their power continue. One of the characteristic features of cavitation erosion is that the removal of metallic material occurs after an incubation period, during which no mass loss, but plastic and elastic deformation of the surface, occurs. The current methods for predicting the amount of erosion damage inflicted upon metallic materials in a cavitating environment are based on the mass loss of material. As a consequence, the erosion-resistance of a material can not be determined until a certain amount of mass loss has occurred. In this study, in order to quickly evaluate the erosion-resistance of metallic materials, the plastic deformation behavior of metallic materials during the incubation period was analyzed. The increment of the surface area was determined from the material surface profile data, measured through a surface roughness meter, to establish a good index for evaluating cavitation erosion development on the surface, as well as the intensity of the cavitation attack. According to the index, the incubation period of cavitation erosion is to be divided into two periods. In the first period, the rate of surface area increment is related to the hardness of the metallic material, and in the second period to the strain hardening exponent. These results can then be applied to the rapid determination of the level of erosion-resistance, in turn leading to a rapid prediction of the service life of the material.