Transcrystallization of polypropylene (PP) on various fibres (Kevlar, PET, carbon and PTFE fibres) was investigated using a polarized optical microscope. The nucleation rate, induction time and nucleation density at saturation were determined at various crystallization temperatures. Results show that the inverse proportion relation between induction time and nucleation rate which is held valid for PTFE and carbon fibre systems is not applicable to Kevlar and PET fibre systems. This is attributed to the different types of nucleation sites resulting from the non-uniformity of surface roughness of Kevlar and PET fibres. Based on the theory of heterogeneous nucleation, the interfacial free energy difference functions Delta sigma of PP on different fibres were determined and compared to that in the bulk matrix. It has been found that Delta sigma(Kevlar) = 3.35 +/- 0.24, Delta sigma(PET) = 5.87 +/- 0.54, Delta sigma(carbon) = 1.14 +/- 0.25, Delta sigma(PTFE) = 0.75 +/- 0.12, and Delta sigma(bulk) = 1.23 +/-0.07 erg cm(-2). From a thermodynamic point of view, PP transcrystallinity is most likely to take place on PTFE fibres due to the lowest value of Delta sigma, which is consistent with the experimental findings. Moreover, induction time, nucleation rate and nucleation density at saturation are also used to characterize quantitatively the nucleating ability of fibres. A simple mechanism, based on thermal stress-induced orientation and relaxation of polymer chains, has been proposed to account for the nucleation of transcrystallization. The topography of the fibre surface plays an important role as well. It is suggested that the presence of small-scale grooves at the fibre surfaces will cause the concentration of thermal stresses and enhance the nucleation process.