Using right materials in right parts leads to the weight reduction of the structures with keepingthe stiffness. Therefore, dissimilar materials have been utilized for vehicle structures. When the dissimilarmaterials are joined together, however, the joints have thermal stress and strain because of atemperature gap, and different coefficients of the thermal expansion. The thermal stress and strainaffect the bonding strength. Thus, the thermal effect should be minimized when the joining strengthis evaluated.In this study, the effect of thermal deformation and thermal stress on the strength of double cantileverbeam（DCB）specimens was investigated analytically and experimentally. A method to reducethe thermal effect with pre-deformation was proposed. The pre-deformation enable us to cancel thethermal stress. Welded specimens between a glass fiber reinforced thermoplastic（GFRTP）and ametal was prepared by this method, and tested using double cantilever beam（DCB）specimens for the measurement of the critical energy release rate. As a result, a welded specimen with no pre-deformationhad larger thermal deformation and stress. However, the welded specimen with pre-deformationhad smaller thermal deformation and stress. Therefore, the true bonding strength was able to bemeasured with the specimen, because the thermal effect on bonding strength was reduced.