MPEG was modified with 1,1'-carbonyldiimidazole, then the activated MPEG reacted with primary amino groups of chitosan. Synthesize the graft copolymer of chitosan and polyethylene glycol in two steps. The structure of the copolymer was characterized by FT-IR and 1H-NMR. It agrees with the PEG content of classical stealth nanoparticles materials. The X-ray diffraction and DSC analysis proved that the crystallinity of the copolymer increased. It is a promising material for the stealth nanoparticles. It is a potential new carrier for the drug delivery systems of long-circulation and solid carcinoma.