Methoxy poly(ethylene glycol)/chitosan graft co-polymers (CS-g-mPEGs) with different degrees of substitution were synthesized by reductive N-alkylation of chitosan with poly(ethylene glycol) aldehyde. The crystalline and thermal properties of CS-g-mPEGs were characterized by wide-angle X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetry (TG). The results indicate that CS-g-mPEG solids represent microphase separation morphology with mPEG crystal and CS domains coexistence and the introduction of PEG on CS improves the thermal decomposition. The hydrodynamic behavior of CS-g-mPEGs in aqueous solution and the influence of NaCl were investigated. The results indicate that the hydrodynamic behavior of CS-g-mPEGs in aqueous solution is significantly affected by the degree of substitution and the concentration of NaCl, which are quite different from that of CS. The results of this paper also certify that CS-g-mPEGs keep the property of complexation with a counter-ion, such as tripolyphosphate, to form nanoparticles through the electrostatic interaction.