Measurements have been made using a polarized optical microscope equipped with hot stages to investigate the transcrystallization of polypropylene on carbon fibres. Both the nucleation rate and induction time at various crystallization temperatures were measured. It has been found that the induction time is inversely proportional to the nucleation rate. Based on the theory of heterogeneous nucleation, the interfacial free energy difference function Delta sigma of PP on carbon fibres was determined and compared to that in the bulk matrix. From a thermodynamic point of view, transcrystallinity is likely to take place on carbon fibres due to a lower value of Delta sigma, 1.14 erg cm(-2), when compared with that in the bulk, 1.23 erg cm(-2).