Abstract: In order to improve the mechanical and barrier properties of polylactic acid( PLA) film,nanocellulose( NCC) was added into it.The microcrystalline cellulose( MCC) was used to prepare NCC with sulfuric acid method,and then the NCC was modified to gain the acetylated nanocellulose( ANCC). The composite films with different ratio of ANCC/PLA were prepared by solvent casting method. The microstructure and morphology of films were observed by TEM,FTIR and SEM. The tensile tester and oxygen transmission tester were applied to analyze the mechanical and gas barrier properties of films.The results showed that,the length of the single nanocellulose fiber was in the range of 100 to 200 nm,and the diameter was in the range of 10 to 20 nm.The Acetylation had little effect on the fiber micromorphology. Compared with pure PLA film,the tensile strength of the composite film with 1%ANCC increased by 29.09%,but the tensile strength was declined with the further increase of ANCC was content.The oxygen transmittance rate of the composite film with 0.5% ANCC was decreased by 42%. The addition of ANCC had no significant effect on the transparence and haze.