Abstract: the monomers 2,6-naphthalenedicaiboxylic acid, terephthalic acid, i sophtha- lic acid and bisphenol were used to prepare liquid crystalline copolyesters by melt condensation and then by solid state polymerization. The effects of temperature and reaction time of solid state polymerization on the melt index of copolyesters have been discussed, with the result that molecular weight of copolyesters can be increased obviously by means of the solid state polymerization. It was suggested that pre-polymerization is chemical reaction control and post-polymerization is diffusion control by analysing curves of melt index vs time. X-ray diffraction spectra indicate that the crystallinity of copolymers has been improved after post-polymerization.