Abstract: A polyester copolymer having an aromatic amine incorporated in the main chain, which is excelent in thermal stability as well as dynamic property, and a process for the production of the polyester copolymer, whereby the existing installation of the prior art can be used as it is, thus resulting in lowering of the production cost, are provided. The polyester copolymer comprises an aromatic dicarboxylic acid, diol and at least one compound selected from the group consisting of aromatic compounds having amino group, represented by the following general formula (I),H.sub.2 N--X--COOR (I)wherein X is an aromatic ring-containing group and R is a hydrogen atom or a hydrocarbon group selected from the group consisting of alkyl groups and cycloalkyl groups.