Damage depth rather than the weight loss of specimen was adopted for representing the extent of cavitation damage to metallic materials. As a result, linear relationship was obtained between the testing duration and the extent of the damage, that is, the damage depth. Furthermore, from this linear relationship a characteristic index was obtained which represented the resistance of material to cavitation attack. In order to represent the extent of damage during the incubation period, where neither weight loss nor damage depth were observed, another parameter named surface increment percentage was introduced. As a result, another characteristic index was obtained, and these indexes coincided with each other not only in physical meaning but also in quantity. Important conclusion drawn from these indexes was that the cavitation erosion mechanism is same in the incubation period as well as in the weight loss period, and that it is also same in laboratory testing apparatuses as well as in actual machines in the field. In the meanwhile, as to the erosion by solid particle impact, the concept of critical impact velocity was introduced to predict the behavior of solid particle at the impact on target material, rolling and skidding. It was made clear that the former causes the damage on the material by plastic deformation and the latter by cutting. This concept was useful to give rationale to the unexpected agreement of the material performance in the field with the result of laboratory test which was conducted under totally different experimental conditions from those in the field: in both cases damage was caused by the plastic deformation process only but without the cutting process at all. The following conclusion was obtained by integrating above research results on cavitation and solid particle impact that the common mechanism generating pure erosion damage in metallic material is plastic deformation.