Manufacturers have been promoting multi-material designs. So, the dissimilar material welding methods are being developed. We focused on heat welding technologies for friction stir spot welding of aluminum alloy and carbon fiber-reinforced plastic. This study investigated the effect of changes to jig constraint of joined members on the fatigue properties of joints. Also the fatigue life estimation was carried out by considering the singular stress at the welding joint interface. As a result, the fatigue strength of joint in a less constrained state is higher than that in a more constrained state. The singular stress intensity at the slit tip was uniformly predicted by the differences in welding parameters of joints.