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Using polarized optical microscopy (POM) equipped with a hot stage, morphological investigations of an isotactic polypropylene (iPP) matrix, induced by a homogeneous iPP fibre and heterogeneous pure/modified nylon 6 fibres, were carried out. With respect to transcrystallization related to heterogeneous nucleation on the surface of the fibre, the nucleation mode was found to be different for iPP fibres and nylon 6 fibres. An iPP fibre can serve as a macroscopic linear nucleus, similarly to the shish‐type structure formed in stress‐induced crystallization, to induce kebab‐like growth of lamellae, whereas numerous closely packed spherulites along nylon 6 fibres resulted in macroscopic transverse growth to form a transcrystallite owing to the limitation along the fibre axis. The difference in nature between these two transcrystallites can be further demonstrated by their optical characters related to the lamellar arrangement inside the transcrystallite. As for homogeneous iPP composites, the formation of transcrystallites results from lattice matching, in consideration of the same chemical structure and lattice parameters between fibre and matrix. The incorporation of calcium chloride into a nylon 6 fibre—to destroy its crystal structure—confirmed the role of lattice matching between nylon 6 fibre and iPP matrix. The addition of atactic polypropylene (aPP) in order to enhance the nucleation ability of the iPP matrix also greatly weakened transcrystallization. Our work demonstrates that transcrystallization is just a matter of competition between interface nucleation and bulk nucleation, namely, if interface nucleation is faster than bulk nucleation, transcrystallization will develop. If not, it will be suppressed. Copyright © 2006 Society of Chemical Industry