Abstract: A random copolyester is described, comprising recurring moieties represented by formula (I), (II), and (III), and having an intrinsic viscosity of not less than 0.5, wherein said recurring moieties of formulae (I) and (II) are existing substantially in equimolar amount and recurring moieties of formulae (I) and (III) are present in a molar ratio of from 95/5 to 5/95 ##STR1## wherein Ar.sub.1 represents a trivalent aromatic group and Ar.sub.2 represents a divalent aromatic group. In formulae (I), (II), and (III), aromatic rings may be substituted with an alkyl group or an alkoxy group having from 1 to 20 carbon atoms, an aryl group or aryloxy group having from 6 to 20 carbon atoms, or a halogen atom. The copolyester obtained is excellent in heat resistance and flame retardance.