PURPOSE: To obtain a granular copolymer having excellent heat resistance and impact resistance, by subjecting an aromatic vinyl and an N-substituted maleimide to graft emulsion polymerization onto conjugated diene rubber to give latex, blending the latex with latex containing no rubber and heating the blend together with a coagulant.

CONSTITUTION: (A) 100pts.wt. latex obtained by subjecting 30W75pts.wt. monomer mixture consisting of 30W80wt.% aromatic vinyl monomer, 5W65wt.% N-substituted maleimide monomer and 0W40wt.% comonomer to graft emulsion polymerization onto 25W70pts.wt. conjugated diene rubber is blended with (B) 0W40pts.wt. latex obtained by subjecting 30W80wt.% aromatic vinyl monomer, 5W65wt.% N-substituted maleimide monomer and 0W40wt.% comonomer, coagulated in the presence of a coagulant, the coagulation system is maintained at 100W190°C and then the powdery copolymer is recovered to give the aimed copolymer.