A fully aromatic polyester based on chlorohydroquinone and terephthalic acid modified with 4,4''-biphenol and isophthalic acid was prepared by polycondensation reaction. These polymers showed thermotropic liquid crystal behavior, which was evidenced by DSC and X -ray diffraction experiments. Thermal treatment of these polymers was performed under various atomospheres as a function of annealing time and the time -temperature relationship of heat treatment process was examined. The experimental results examined by DSC showed that the peak temperatures are shifted progressively to higher temperature and the peak areas are increased by extending the annealing time. As a result of solid -state polymerization, the melting behavior of heat treated specimens depended significantly on the environmental conditions X-ray results revealed that a heat treated specimen has an orthorhombic unit cell (a ; 6.4 A, b : A, c ; 23.1 A)and further experiments should be under-taken to confirm and refine the tentatively proposed cell parameters.