The present invention relates to a novel copolymerized composition characterized in that it comprises (A) 100 parts by weight of a copolymerized polyester containing (a) an aromatic dicarboxylic acid unit composed mainly of a terephthalic acid unit, (b) a glycol unit composed mainly of an ethylene glycol unit, (c) a polytetramethylene glycol unit, (d) a polyethylene glycol unit and (e) an aliphatic dicarboxylic acid unit having 9 or more carbon atoms, (B) 5 to 150 parts by weight of a reinforcing substance and (C) 1 to 10 parts by weight of a metal salt of an ionic copolymer.The copolymerized polyester compositions of the present invention retain excellent heat resistance and rigidity that the glass fiber-reinforced polyethylene terephthalate compositions possess originally and are considerably improved in terms of low-temperature moldability, toughness, alkali resistance and like other factors.