The quality of thermoplastic injection molded parts is mainly determined during the post-filling process. To enhance the control capacity over the pose-filling process, adding compression to the conventional injection molding process opens a flexible channel for pressure control. This study experimentally investigated the effects of adding compression on the thermal-mechanical history and on the quality of molded parts. An instrumented molding machine with a compression mechanism was used. Molds for rectangular plate and circular disk were equipped with a movable cavity wall and shut-off mechanism. The thermomechanical history during the injection compression molding was identified with the aid of a PC-based data acquisition system. The quality of parts was evaluated by flatness, deviation from mold dimensions, and birefringence. It has been found that the thermomechanical history of the melt under injection compression molding follows a quadrilateral-shaped path over the p-v-T diagram. The control capacity over post-filling is enhanced. It thus can produce parts with better dimensional control with less frozen-in orientation than those from conventional injection molding.