Abstract: Polyhedral oligomeric silsesquioxane/ploy (L-Lactic acid) (POSS/PLLA) composites were prepared by blending POSS and PLLA with different molecular mass in dichloromethane solvent. The crystallization morphology, crystallization behavior and thermal properties of POSS/PLLA composites were studied by polarizing microscope (POM), DSC, XRD and TGA. The results show that the crystallization temperatures of POSS/PLLA composites increase to about 110℃ and their crystallization ability increases. The initial thermal decomposition temperature and the terminate thermal decomposition temperature of the POSS/PLLA composites increase and the temperature corresponding to the maximum decomposition rate is between 373℃ and 379℃, which indicates that their thermal stability is improved. When the content of POSS is lower, the nucleation plays a dominant role. When the content of POSS is higher, it hinders the movement of PLLA chains. And the blocking effect of POSS is stronger for high molecular weight PLLA. The cross extinction phenomenon and the ring band crystal morphology appear in POSS/PLLA composites and PLLA are seen obviously during isothermal crystalization. The spherulite in the cooling process produces cracks, which is related to the brittleness of PLLA.