In the emulsion polymerization of a monomer mixture comprising an aromatic vinyl compound, a vinyl cyanide compound and if desired, other vinyl compounds copolymerizable therewith in an aqueous medium, the vinyl cyanide compound is further added to the polymerization system in a controlled velocity during the substantial period of polymerization from the initial stage. When the copolymer thus obtained is incorporated into, for example, an acrylonitrile-butadiene-styrene terpolymer composition, thermoplastic resin compositions not coloring upon molding are obtained with a good impact resistance and heat resistance.