Abstract: Poly (lactic acid) (PLA) graft copolymer (mPLA) was prepared by graft copolymerization using maleic anhydride and butyl acrylate as comonomers. Then the mPLA was used as compatibilizer to prepare cellulose nanocrystal (NCC)/PLA composites by solution casting method. Effects of mPLA on the structure and properties of NCC/PLA composites were investigated by SEM, DSC, TG, wide angle X-ray diffraction(WXRD), mechanical testing and degradation property test. Results show that the application of mPLA in NCC/PLA composites improves compatibilization between NCC and PLA, and promotes dispersion of NCC in the PLA matrix. More finely dispersed NCC enhances the crystallization nucleation of PLA, resulting in lower crystallization temperature and higher crystallinity. Mechanical properties of NCC/mPLA/PLA composite increase then decrease with increasing mPLA content, when the content of mPLA is 8%, the tensile strength and elastic modulus increase about 30.2% and 41.4% respectively in comparison with those of composite without mPLA. The hydrophilic NCC accelerates degradation of NCC/PLA composite, when mPLA is added, the degradation rate of NCC/PLA composite slowes down but is still higher than that of pure PLA.