PURPOSE: To prepare a polyester which is excellent in melt moldability and mechanical properties and gives a molecularly oriented fiber by forming the polyester with two specific dicarboxylic acid residues and specifying the intrinsic viscosity of the polyester.

CONSTITUTION: The title polyester is a copolyester in which the dicarboxylic acid residues constituting the polyester are of formulae I and II in the molar ratio of 30/70-10/90 and the diol residue is composed of that of formula III and which has an intrinsic viscosity of 0.4 or more as measured in a p- chlorophenol/1,1,2,2,2-tetrachloroethane solvent mixture (40/60 by weight). The polyester is a material which is crystalline and has an excellent melt moldability, and hence can widely be used as fibers, films and moldings. Further, the fiber, in particular, has an excellent moldability and spinning and oriented crystallization characteristics and also excellent mechanical properties, thermal resistance and chemical resistance and therefore is useful in general industrial applications.