A series of poly(p-oxybenzoate-co-p-phenylene isophthalate)s were prepared by melt polycondensation of p-acotoxybenzoic acid, p-phenylene diacetate, and isophthalic acid followed by solid-state polymerization. The solid-state polymerization not only increased the molecular weight, but also annealed the copolyesters. The thermal stability, liquid crystallinity, thermal properties, and solid-state crystallinity of these copolyesters were investigated by TGA, polarized microscope, DSC, and wide-angle X-ray diffraction, respectively. It was found that copolyesters with HBA content of 50, 60, and 70 mol % showed explicit melt birefringence at 370°C and thus exhibited liquid crystalline state. The effect of annealing during solid-state polymerization on the thermal properties was discussed. The effect of copolymerization on their crystallinity was interpreted with X-ray diffraction qualitatively.