In this research, to obtain Low molecular water-soluble chitosan(LMWSC) with various molecular weights, the crude COS was separated according to its molecular weight by using ultrafiltration technique. LMWSC with a free amine group was prepared by the movel salts-removal method described. Absolute molecular weights(Mw), and molecular weight distribution of COS were determined by gel permeation chromatograph(GPC). Degree of deacetylation of LMWSC was determined by 1H-NMR and 13C-NMR spectra the absorption band by carboxyl group derived from lactic acid disappeared than that of COS.