In order to synthesize the high molecular weight poly(lactic acid) [PLA], Hexamethylene diisocyanate [HDI] was used as a chain extender connecting the terminal group of polymers. Poly(lactic acid) of high molecular weight, 76000 (M(w)) and 33000 (M(n)), was obtained by using antimony trioxide catalyst after chain-extending with HDI. Without chain-extending, the molecular weight of PLA was about 7000 (M(n)). T-m in the 2nd-scan DSC thermogram was not found when PLA was chain-extended with HDI. In order to check the degree of crystallization of polymers, annealing of these polymers was carried out at 120 degrees C. Peaks of XRD (X-Ray Diffraction) were sharpened as the duration of annealing was lengthened. The analysis of polymers reacted with HDI by H-1-NMR showed the broad peaks at 1.32 and 3.14 ppm assigned to HDI units. The molecular weight of polymers increased with the increase in the mole ratio of -NCO/-OH.