A series of co [poly (ethylene terephthalate-p-oxybenzoate)] thermotropic copolyesters with various compositions were prepared by the copolymerization of either poly (ethylene terephthalate) polymer (PET) or its oligomer (OET) as moiety (II) and p -acetoxy-benzoic acid (POB) as moiety (I). The polymeric products obtained were then subjected to solid-state polymerization. The glass and melting transitions of the copolyesters obtained have been studied by differential scanning calorimetry (DSC). Effects of composition and solid-state polymerization on DSC are discussed. The melting point of copolyesters possesses a higher value if the PET polymer is used as PET moiety in the copolyesters. In the DSC scan of the POB rich composition, the endothermic peak shows obscurely and enthalpy of fusion becomes small due to the change in the crystalline morphology from isotropic to anisotropic. In general, the melting point of copolyesters is increased by the solid-state polymerization reaction. It is also found that both the melting temperature and enthalpy of fusion of the copolyesters can be increased simultaneously by the solid-state polymerization reaction only when the composition of POB / PET is 80 / 20. This phenomenum at this composition may be attributed to the annealing effect caused by the randomness of two moiety units on the backbone chain of copolyesters and the increased molecular weight as well.