Nanocrystalline cellulose (NCC)/poly (lactic acid) (PLLA) composite membranes were prepared by the solution casting method. Physical and chemical modifications were performed to improve the compatibility of NCC and PLLA. The results indicated that the NCC dispersibility of the composite membranes with chemical modification were superior to that of the composite membranes with physical modification. Moreover, the chemical modification NCC not only had a large effect on the nucleation and growth of the crystals, but also affected the crystal forms of PLLA. This was because chemical reactions took place between the silicone of silane coupling agent (KH-570) and the hydroxyl groups of NCC during blending. The chemical modification NCC could be dispersed stably in the PLLA matrix, and it played the role of a nucleating agent.