The POB/PBT copolyesters, designated B28, B46, B64, and B82, were prepared from p-acetoxybenzoic acid (PAB) and poly(butylene terephthalate) (PBT). The polymeric products obtained were then ground and subjected to solid-state polymerization under vaccum for 4 h. The melting and crystallization behaviors of these copolyesters haven been studied by differential scanning calorimetry (DSC). In the DSC scan of the POB-rich composition, the endothermic peak shows obscurely, and enthalpy of fussion becomes small due to the change in the crystalline morphology from isotropic to anisotropic. In general, the melting point of the copolyester is increased by the solid-state polymerization reaction. Also, thermogravimetric analysis (TGA) were performed with these samples obtained. It was found that the decomposition temperature (T-d) is increased as the POB content is increased. Effects of composition and solid-state polymerization on the decomposition temperature of copolyesters are also discussed. The crystalline morphology of copolyester was investigated with a Zeiss polarized optical microscope. It was found that the POB/PET copolyesters with 60 mol % POB was shown to be highly anisotropic.