This paper is mainly concerned with the dynamic structure of high-speed water jets, the damage process of metallic materials and a microscopic comparison of cavitation damage. The experimental conditions were an injection pressure of 30 to 90 MPa and a nozzle inner diameter of about 1 mm. Materials tested included aluminum, mild steel and stainless steel. The instantaneous behavior of the impact pressure was measured using a pressure transducer. Comparisons among stroboscopic observations for the structure of water jet, axial variation of the impact pressure and mass loss of the material caused by the jet impact showed that an axial location where the water jet begins to break up is close to the location of the maximum mass loss of the material. We have made a detailed comparative observation of the damage process for a water jet and for the cavitation (magnetostrictive vibratory method) using a scanning electron microscope and found some distinct differences in the damage process.