A process for preparing .alpha.-methylstyrene/acrylonitrile copolymer by suspension or bulk polymerization of .alpha.-methylstyrene and acrylonitrile with at least one member selected from the group consisting of styrene, chlorostyrene, para-methylstyrene, t-butylstyrene, acrylic ester and methacrylic ester in the presence of a difunctional organic peroxide having a 10 hour half-life temperature of 60.degree. to 120.degree. C. and capable of producing a t-butyl radical, at a polymerization temperature of 80.degree. to 135.degree. C. The process produces the copolymers having excellent transparency, heat resistance and strength in a high conversion in a short time.