The effect of flax fibre reinforcement on the crystallisation behaviour of polypropylene (PP) was investigated using a hot-stage polarising optical microscope. To follow the crystallisation kinetics, crystallisation temperature and time were varied. At crystallisation temperatures between 130 and 138 A degrees C the most uniform and thickest transcrystalline layers were formed. The effect of transcrystallisation on the interfacial shear strength (IFSS) in micro-composites was studied by the fibre pull-out test method. It was found that the IFSS of the PP/flax system is slightly decreased with the presence of a transcrystalline interphase. Finally, the influence of the formation of a transcrystalline zone on the macromechanical properties of compression moulded PP/flax composites was studied.