In conventional injection molding, due to the differences in mechanical design for mold core and cavity sides, balanced cooling and symmetrical mold temperature are not easily achieved resulting in great influence on part warpage. The new development trends for injection molding process include two component injection molding, in-mold decoration, etc. In these molding processes, one side of mold cavity is subjected to significant unbalanced heat transfer even though the cooling channel and coolant operation conditions are the same for both cavity and core sides. Therefore, to solve the warpage under an unbalanced molding cooling and asymmetrical mold temperature condition becomes an important topic for the new injection molding era. In this study, the innovative Gradual Thickness Variation (GTV) method was utilized on the part thickness design combing different cavity and core temperature settings in order to reduce the warpage of IMD parts and common plastic parts. Part thickness reductions were 20%, 30% and up to 40% of the original part thickness. The results reveal when the part thickness reduction percent increases, the warpage increased, which means we can solve warpage value in the injection or IMD part.