PURPOSE: To provide the titled compsn which gives moldings having excellent heat resistance, mechanical properties and color tone, by filling a thermoplastic high-molecular resin with specified aluminum hydroxide.

CONSTITUTION: A thermoplastic high-molecular resin (A) such as an unsaturated polyester resin, an acrylic resin, an epoxy resin, PVC, a polyolefin resin, etc. is blended with 10W90wt% gibbsitic Bayer-process aluminum hydroxide (B) having an average particle size of 0.5W15μ, an N2 gas adsorption surface area of not lower than 2.8m2/g, a total Na2O content of not higher than 0.08wt% and a whiteness degree of 97 or higher, whose surface is optionally treated with a silane coupling agent to such an extent that the content of said silane coupling agent is 0.5W2wt%.