True stress–true strain curve of JIS AD12.1 (A383.0) alloy was obtained by tensile test in the partial solidification state for predicting residual stress and distortion of aluminum castings. It was found that tensile strength in true stress–true strain curves decreased with increasing temperature and decreasing strain rate. In addition, most of true stress–true strain curves showed constant stress with increasing strain. Thus, the constitutive equation taking account of strain-rate-dependence and stress saturation in the partial solidification state should be used in the thermal-stress analysis during solidification. Material constants of Norton creep law of JIS AD12.1 alloy in the partial solidification state were successfully obtained for the first time in the world.