O-Acetyl-chitin was prepared in a lithium chloride-N,N-dimethylacetamide (LiCl/DMAc) system with high yield. The degree of acetylation was controlled with the amount of acetic anhydride. The obtained O-acetyl-chitin swelled not only in polar organic solvents. Such as N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO), but also in water. O-Acetyl-chitin powder was graft-copolymerized to methyl methacrylate without catalyst. Graft copolymerization was initiated by the photolysis of the O-acetyl-group and the polymerization in a Quartz tube proceeded easier than in a Pyrex tube. The addition of DMSO in the reaction system did not always accelerate the-graft copolymerization. Effects of various factors such as monomer concentration, degree of substitution of the O-acetyl-group and polymer concentration were also investigated.