Peening method using cavitation impacts have been developed. In this peening method, cavitation bubbles were generated by a cavitating jet. In order to increase peening intensity, a low speed water jet was injected around a cavitating jet, as the water jet swept away residual bubbles, which weaken the cavitation impact. In the present paper, improvement of fatigue strength of stainless steel by using a cavitating jet with an associated water jet was investigated compared with the results of conventional cavitating jet. It was concluded that the cavitating jet with an associated water jet can peen the stainless steel more effectively compared with the conventional cavitating jet.