Cavitation impacts of ultrahigh-speed water could lead to stresses erosion and other damage on the surface of hydro machinery such as pumps, turbines and valves. On the other hand, the cavitation impacts can be used to modify the residual stress on surface of materials as a shot peening method. Recently, a technology to modify residual stress on a plate by using of cavitation tunnel and a cavitation water jet in air has been reported. A high level of compressive residual stress has been applied on surface of materials by using water cavitation jet. In the present paper, residual stress of a helical gear of SCR420H steel was measured by means of an X-ray diffraction method, before and after peening by water cavitation jet. In order to obtain high-level compressive residual stress, optimum peening time and standard distance from jet nozzle to surface of the gear were investigated by cavitation jet tests. These results prove that residual stress on gear surface can be improved by using cavitation jet technique.