A process for preparing .alpha.-methylstyrene/acrylonitrile copolymers by suspension or bulk polymerization of .alpha.-methylstyrene and acrylonitrile with or without other vinyl compounds such as styrene, methyl methacrylate, vinyl toluene and t-butylstyrene in the presence of a difunctional organic peroxide, preferably a difunctional organic peroxide having a 10 hour half-life temperature of 60.degree. to 110.degree. C., at a polymerization temperature of 80.degree. to 120.degree. C. The process produces the copolymers having excellent transparency, heat resistance and strength in high conversions in a short time.