A series of thermotropic liquid crystalline copoly(ester-imide)s, which formed nematic phase, were synthesized from 2,6-diacetoxy naphthalene and the varying amount of the two n-(ω-carboxyalkylene) trimellitic imides. They had the lower crystal-nematic transiton temperatures and relatively slower crystallization rate than homopoly(ester-imide)s. It seemed that structural irregularity due to the random monomeric sequence made the crystallization of these copolymers difficult. A qualitative explanation of the hysterisis of TLCPs required significant amounts of identical monomer sequences to initiate crystallization upon cooling. Consequently, the viscoelastic properties of homopolymers showed the strong dependence of the crystallization rate and residual crystallites in the nematic phase of which the origin is the relatively regular monomeric sequence in polymer chain. Copoly(ester-imide)s with irregular monomeric sequence showed little effect of thermal history because these copolymers had a very low degree of the crystallinity and the rate of crystallization was very slow so that almost all portion of the nematic phase was kept after solidification process. Fibers were spun from copoly(ester-imide) as a function of draw ratio at its melting temperature. It also contained no crystallite and its molecular orientation and mechnical properties increase very rapidly with draw ratios.