Abstract: Carbon fiber/poly (lactic acid) (CF/PLA) composites were prepared by using CF as filler. The mass ratio of CF(CF:PLA) were 1%, 3%, 5%, 10% and 15%. Crystallinity and rheological properties of PLA and CF/PLA composites were investigated. The results show that, CF/PLA composites with 1% and 3% CF can improve the overall crystallinity of PLA at cooling process due to the heterogeneous nucleation mechanism. The intensity of XRD diffraction peaks gets stronger. The crystallization temperature and crystallinity respectively of CF/PLA composites increase to 112.5℃ and 30.7%. And the rheological properties are similar to those of pure PLA. When the mass ratio of CF increases to 5%, which is the "percolation threshold", the viscosity of CF/PLA composites gets a sharp rise. The movements of molecular segments are limited, leading a decrease in crystallinity of PLA. When the mass ratio of CF is 15%, the crystallization temperature of CF/PLA composites reduces to 93.1℃ and the crystallinity is only 2.5%.