Partially N-acetylated chitosan was hydrolyzed by the cheap, commercially available, and efficient cellulase. The products, with different molecular weight, were comparatively investigated by GPC, FT-IR, XRD, and NMR. The results show that the decrease of molecular weight led to transformation of crystal structure and increase of water-solubility, but the chemical structures of residues were not modified. Superoxide anion radical and hydroxyl radical quenching assay were used for the evaluation of free radical scavenging activity of cellulase-treated chitosan in vitro. Low molecular weight chitosan (LMWC3, M-w 1.7 x 10(3)) exhibited high scavenging activity against free radical. It scavenged 79.3% superoxide radical at 0.1 mg mL(-1). At 2.0 mg mL(-1), scavenging percentage of initial chitiosan, LMWC1 (M-w 27.3 x 10(3)), LMWC2 (M-w, 5.9 X 10(3)), and LMWC3 (M-w 1.7 x 10(3)) against hydroxyl radical was 14.3%, 33.1%, 47.4%, and 65.9%, respectively. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 545-550,2009