A polyester composition comprising 0.005 to 10% by weight compound oxide particles having a volume average particle meter of 0.005 to 2 .mu.m, a relative standard deviation .sigma. of the particle diameter of not more than 0.3, and composed mainly of silicon (Si), aluminum (Al) and at least one alkali metal (M), wherein the contents of the silicon, aluminum and alkali metal satisfy the following requirements: 10 wt. %.ltoreq.Si.ltoreq.45 wt. %, 3 wt. %.ltoreq.Al.ltoreq.30 wt. % and 0.5 wt. %.ltoreq.M.ltoreq.20 wt. %, respectively. The polyester composition, particularly when formed into films, has excellent surface evenness, running ability and abrasion resistance, and such films are suitable particularly for magnetic recording media and other applications.