Two types of semiflexible mesogenic random copolyesters which contained both nematic and smectic type repeating units within the main chain were prepared and the thermal properties as well as the mesomorphic structures were characterized by differential scanning calorimetry and by polarizing microscope. The first type of copolymer (copolymer I) contained two repeating units which differed in the rigid mesogenic groups and the second (copolymer Ⅱ) contained repeating units which differed in the length of flexible segment. Copolymerization disrupted the structural regularity of the crystal, lowered the crystal-mesophase transition temperature, and destabilized the molecular order of the smectic phase. These effects were more pronounced for copolymer Ⅱ. However, the isotropization temperature was changed only slightly with the incorporation of the other mesogenic component. The crystal melting temperatures of both copolymers exhibited aeutettic behavior. A smectic to nematic transition, which was not observed for the homopolymers, occurred in the range of 0.4-0.6 mole fractions of the smectic units for copolymer I and 0.5-0.85 for copolymer ll. The molecular order of the nematic phase was slightly increased as the smectic units were incorporated, while those of the smectic phase became more disordered upon addition of the nematic units.