A process for dimensionally stabilizing an article molded from a synthetic resin material, such as PVC. The molded article is subjected to a first cooling treatment to cool the article at a temperature lower than the glass transition temperature Tg of the synthetic resin followed by a heating treatment at a temperature higher than the glass transition temperature Tg and then is subjected to a second cooling treatment again at a temperature lower than the glass transition temperature Tg. Following such treatment steps, the molded article, such as side molding for automobiles, have less variation in dimensions and less shrinkage after molding in use.