We have prepared main-chain liquid crystalline copolyesters made from biphenyldicarboxylic acid as a mesogenic component and a certain mixture of 2-methylbutanediol and hexanediol as spacer components. These copolymers exhibit the novel polymorphism with the smectic A as a higher temperature mesophase and the smectic C phase as a lower temperature one. The transition behavior of smectic A to smectic C has been examined by X-ray diffraction method and the layered structure of smectic C phase has been described by a comparison with the preceding smectic A structure.