Abstract: The needle rod-like cellulose whiskers (NCW) with nano size and certain length-diameter ratiowere prepared by acid hydrolysis, and then were surface modified with poly (L-lactic acid) (PLLA) segments through thering-opening polymerization of L-lactide initiated by surfacehydroxyl groups to obtain grafted cellulose whiskers(g-NCW). PLLA membrane and NCW/PLLA and g-NCW/PLLA composite membranes with different ratios were prepared by solution casting method. Morphologies and properties of the modified and unmodified NCW were observed, and the morphologies, crystallization properties, thermostability, hydrophilicity/hydrophobicity and tensile properties of the composite membranes were studied too. The results show that the morphology and crystallization property of NCW change little after graft modification, but the dispersion of g-NCW in ethanol and PLLA solution is improved significantly. The grafting percentage of PLLA segments on surface of g-NCW is 23.61% as the ratio of amount of substance of NCW to L-lactide is 1∶5. The crystallization rateof PLLA matrix increases significantly due to the heterogeneous nucleating effect of the NCW and g-NCW. In addition, the hydrophilicity and thermostability of material are improved by the introduction of the whiskers.When adding some content of NCW and g-NCWto PLLA, the strength and toughness of the PLLA matrix are evidently improved. With the content of whiskers increasing, the tensile strength and breaking energy increase first and then decrease for NCW/PLLA and g-NCW/PLLA composite membranes, as the mass fraction of NCW and g-NCW is 5%, the highest tensile strength and breaking energy of 22.02 MPa and 20.01 MPa, 102.39 J/m³ and 117.83 J/m³ are obtained for NCW/PLLA and g-NCW/PLLAcomposite membranes respectively. Due to the excellent dispersionof g-NCW in matrix and interfacial adhesion, the tensile strength and toughness of g-NCW/PLLA composite membranes are obviously superior to those of the pure PLLA and NCW/PLLA membranes.