Abstract: The composites Polylactic Acid （PLA）／Cellulose Acetate Butyrate （CAB）was prepared by casting method，the glass-transition temperatures of different components of composites were measured by Differential Scanning Calorimeter （DSC），whereas the form and isothermal crystallization of different components under molten state was observed by Polarized Light Microscope （PLM）.The results showed that PLA and CAB were incompatible，and CAB exerted an inhibiting effect on the nucleation of PLA.