PURPOSE: The titled composition that is obtained by mixing a methylstyrene/ acrylonitrile copolymer with a specific graft copolymer, thus having excellent balance between heat resistance and mechanical properties such as impact strength.

CONSTITUTION: The objective composition is obtained by mixing (A) 50W 95pts.wt. of a copolymer from 20W90wt% of α-methylstyrene, 5W45wt% of acrylonitrile and 0W60wt% of copolymerizable monomers with (B) 50W5pts.wt. of a graft copolymer which is prepared by polymerizing 60W20pts.wt. of a monomer mixture consisting of (i) 30W90wt% of p-t-butylstyrene, 5W45wt% of acrylonitrile and 0W50wt% of copolymerizable monomers in the presence of (ii) 40W80pts.wt. of a rubber-like polymer.