Co polymerization of chitosan selectively grafted by polyethylene glycol was studied in this paper. The graft copolymers can be expected to be used as DNA delivery vectors because of the improving hydrophilicity of chitosan and reserving the amino groups on chitosan chains. Chitosan was selectively grafted by monomethoxy polyethylene glycol(mPEG-OH), which contains a hydroxyl group combining with hexamethylene diisocyanate(HDI) to form a novel macro monomer namely monomethoxy polyethylene glycol isocyanate (mPEG-NCO) containing a isocyanate group with higher chemical activity in ethyl glyoxalate solution absolutely without water. The selective grafted co polymerization of Chitosan with mPEG-NCO was conducted under heterogeneous conditions as suspension in dimethylformamide. The hydrophilic copolymers of chitosan were prepared by condensation reaction of isocyanate group on mPEG-NCO with hydroxy groups on chitosan chains because amino groups on chitosan chains were protected by complexion formation with copper ions. The effect of reaction condition on the grafting extents was discussed. Swelling properties of mPEG-g-CS were researched. The graft copolymer mPEG-g-CS was characterized by the infrared spectra. The result showed that the copper ions were very effective to protect amino groups from condensation reaction. The swelling level in water increases with adding of grafting ratio. The maximum swelling degree was up to above 132% when the grafting ratio was about 270%. The graft copolymer can be soluble partially in pure water.