PURPOSE: To improve the heat resistance of a synthetic resin by mixing it with a silicone oil and a silicone-graft polyester comprising a specific polyester main chain and methylpolysiloxane side chains.

CONSTITUTION: A silicone-graft polyester having a transition point of 40°C or higher is formed by subjecting a polycarboxylic acid and/or its deriv. and a polyhydric alcohol to polycondensation at 150-280°C to form a main chain polymer such as a polyester polyol having a wt. average mol.wt. of 2,000-100,000 and reacting the main chain polymer with a side chain polymer such as an isocyanate-contg. organosiloxane represented by the formula (wherein R1 is a divalent org. group; L is 0-50; m is 2-3; and n is 1-130) in an org. solvent at 50-100°C. Then, 100 pts.wt. synthetic resin (e.g. a PP resin) is melt mixed with 0.1-50 pts.wt. silicone oil having a viscosity (25°C) of 50-100,000cSt (e.g. a dimethylsilicone oil) and 0.1-20 pts.wt. the silicone-graft polyester and kneaded to give a highly heat-resistant synthetic resin compsn.