A polyester resin composition which comprises(A) polyethylene terephthalate or a copolyester containing at least 80 mol % of ethylene terephthalate repeating unit.(B) a metal salt of a copolymer composed of .alpha.-olefin and .alpha.,.beta.-unsaturated carboxylic acid,(C) a polyvalent oxazoline compound of the formula: ##STR1## wherein R is an aliphatic or aromatic hydrocarbon group such as alkylene or arylene having 1 to 24 carbon atoms optionally substituted with lower alkyl having 1 to 6 carbon atoms or aryl having 6 to 19 carbon atoms; n is 0 or 1; X and Y are hydrogen or 2-oxazoline group, when n is 1, or no X and Y are present, when n is 0; and all oxazoline may be substituted with lower alkyl having 1 to 6 carbon atoms or aryl having 6 to 19 carbon atoms, and optionally(D) a polyester elastomer.