An improved polyester exhibiting melt anisotropy and excellent thermal stability is provided which consists essentially of the structural units (I) to (III) wherein: ##STR1## wherein Ar.sub.1 is 1,4-phenylene, Ar.sub.2 is a mixed system of 2,6-naphthalene and 2,7-naphthalene, and Ar.sub.3 is an aromatic ring containing 6 or more carbon atoms, the unit (I) accounting for 15 to 90 mole percent and the unit (II) for 20 to 70 mole percent of the total amount of the structural units, and the percentage of 2,7-naphthalene to the total amount of Ar.sub.2 in the unit (II) being between 10 and 90 mole percent.