A new type of biodegradable amphiphilic graft copolymers, PEG-g-hexanoyl chitosan, was synthesized by a facile scheme. The self-assemble properties of the copolymers were studied by TEM, fluorometry and dynamic light scattering. It was found that spherical nanoparticles of the copolymers could be formed through dialysis method. With the increase of PEG content in the copolymers, the average diameter of the nanoparticles decreased from about 180 to 40 nm. PEG fraction in the copolymers has little effect on the copolymer CAC. The micellization of the copolymers was strongly dependent of the medium pH. When pH was lowered from 7.2 to 6.8, the average diameter of the nanoparticles dramatically changed from about 180 to 60 nm.