Poly(ethylene terephthalate)/p-hydroxybenzoate (PET/PHB) copolymer materials have relatively low melt viscosity because of the 60 mol % PHB material having a value that is lower by approximately two orders of magnitude than the value for PET homopolymer. The structure development during melt spinning and thermal treatment (annealing) of liquid crystalline copolyesters, with a rigid backbone structure, were analyzed through the density, birefringence, X-ray diffraction, DSC, dynamic viscoelasticity, and tensile testing. As the take-up velocity increased, the birefringence of PET/PHB as-spun fiber increased, which indicated that it directly influenced the initial modulus and specific stress. The lateral packing of PHB molecular chain in a copolymer was shown to be loosened in the course of thermal treatment. The thermal treatment slightly increased the crystal orientation factor, whereas total molecular orientation was decreased by annealing. (C) 2004 Wiley Periodicals, Inc.