Polarizing microscopy and differential scanning calorimetry (DSC) were used to study the effect of carbon fiber on the crystallization of polypropylene (PP). It is shown that the surface of carbon fiber displays a high propens ity to nucleate PP crystallization. The nuclei density at the surface of carbon fiber is much higher than that in the bulk area, and the resulting radial growth produces a type of transcrystalline structure, and the characteristic temperature of crystallization increases with carbon fiber concentration. By the study of the crys-tallization kinetics it is concluded that the crystallization in spherulitic in nature,carbon fiber promotes the crystallization of PP by decreasing it's crystalline surface free energy.