This invention is a polyester resin composition comprising 100 parts by weight of a resin composition consisting of 60 to 95 wt % of (a) a polyester resin and 5 to 40 wt % of (b) an olefin-based resin, and 0.5 to 30 parts by weight of (c) one or more resins selected from polyphenylene sulfide resins and liquid crystal resins, wherein said olefin-based resin (b) is composed of (b-1) a functional group-containing olefin copolymer having at least one kind of functional groups selected from carboxylic acid groups, carboxylic anhydride groups, carboxylic ester groups, metal carboxylate groups and epoxy groups and (b-2) an ethylene/.alpha.-olefin copolymer obtained by copolymerizing ethylene and an a-olefin with 3 to 20 carbon atoms; and the polyester resin (a) forms a continuous phase, while the olefin-based resin (b) is dispersed as particles with an average particle size of 0.01 to 2 .mu.m in the composition. This invention provides a polyester resin composition having excellent flexibility and impact resistance in a low-temperature atmosphere, also having excellent flowability and chemicals resistance, and suitable for injection molding.