Several experiments were run to determine the relations between shot peening variables (shot size, shot velocity, impact angle, peening time) and surface residual stress. The surface residual stress was measured by X-ray diffraction method. The results are as follows : (1) The surface residual stress increases with peening time and then approaches a saturated value (-350-400 MPa) before full coverage. (2) Surface residual stress after full coverage depends slightly upon the shot peening conditions. (3) Surface residual stress shows the size effect to the thickness for the thin sheet of specimen. (4) Residual stress induced in the affected layer exists together with the non-affected zone and falls to zero where the depth of affected layer equals to the thickness of specimen.