PURPOSE: To obtain a heat stabilizer which gives a thermoplastic resin compsn. excellent in heat resistance, delamination resistance, and flowability by copolymerizing an epoxidized vinyl compd. with another copolymerizable vinyl compd.

CONSTITUTION: A heat stabilizer is prepd. by copolymerizing 0.01-50wt.% epoxidized vinyl compd. of formula I (wherein Ar is a 9-23C arom. hydrocarbon group having at least one group selected from glycidyloxy and glycidyl groups; and R is H or CH3)(e.g. a compd. of formula II) with 50-99.99wt.% other copolymerizable vinyl compd. (e.g. one contg. at least 20% α-methylstyrene and/or β-isopropylnaphthalene). The stabilizer is compounded into an olefin resin to give a thermoplastic resin compsn. excellent in heat resistance, delamination resistance, and flowability.