Abstract: Bagasse cellulose extracted by method of combination of acid and alkali hydrolyzed to yield bagasse cellulose microcrystal (BCMC) using acid hydrolysis. The biodegradable composite films were prepared by using the bagasse cellulose microcrystal as filler and poly lactic acid (PLA) as the polymeric matrix. The structure, the mechanical properties and the thermal stability of the composite film were tested. The results show that the BCMC distributed well in the PLA matrix without obvious aggregation, adding the degree of crystallization of composite films. When the mass fraction of BCMC was 5%, the optimized integrated performance of composite films could be gained. The initial decomposition temperature increased 30.73 ℃, the tensile strength increased by 50.98% and the elongation at break increased by 16.25% compared with those of the pure PLA.