PURPOSE: To efficiently obtain, in high productivity, a thermoplastic polyester composition excellent in homogeneous dispersibility of filler, useful as fiber and film by kneading a thermoplastic polyester with a specific filler composition.

CONSTITUTION: (A) A thermoplastic polyester is kneaded with (B) a composition comprising (B1) the compound of formula: Z-[O-(AO)p-R]2 (Z is an aromatic dihydroxy residue; p≥2), (B2) the terpolymer of formula: (B is an alkyleneoxide group; M is a maleic anhydride residue; S is styrene residue; A is a 2-3C alkylene; m and n are each a positive integer) and (B3) a filler (for example kaolin or calcium carbonate) to give the objective polyester composition.