Thermoforming of polypropylene/CaCO3 composite sheets has become an important process in industry because of their low cost and good formability. However there are some unsolved problems that confound the overall success of this technique. Nonuniform thickness distribution caused by inappropriate mold design and processing condition is one of them. An L＇18 experimental matrix design based on the Taguchi method was conducted to optimize the thermoforming process of CaCO3 filled polypropylene composites. For the factors selected in the main experiment, the assist plug＇s displacement and velocity were found to be the principal factors affecting the thickness distribution of thermoformed composites.