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A general feature of the morphogenesis of semicrystalline polymers is the regular stacking of chainfolded lamellar crystals alternating with amorphous layers. This stacking s characterized by an intercrystalline long period L p including the crystalline lamella and amorphous layer thicknesses. The stacking long period is in the range of a few nanometers to a few tens. When crystallization is performed from a quiescent melt, the resulting microstructure is isotropic with a spherulitic arrangement of the lamella stacks as a result of the radial growth of the crystals from a common multiple nucleus often designated as axialite [1]. The size scale of spherulites ranges from a few to several hundred microns. Upon plastic deformation at large strains, isotropic semicrystalline polymers transform into fibrillar materials that aroused a great interest from both academic and industrial standpoints owing to their remarkable mechanical performances [2-6] to be used as ropes, textile fibers for high performance tissues and fabrics, reinforcing fibers for polymer-based composites, etc. Abstract. The influence of crystalline microstructure and molecular topology on the strain-induced fibrillar transformation of semi-crystalline polyethylenes having various chemical structures including co-unit content and molecular weight and crystallized under various thermal treatments was studied by in situ SAXS at different draw temperatures. The long period of the nascent microfibrils, L pf , proved to be strongly dependent on the draw temperature but non-sensitive to the initial crystallization conditions. L pf was smaller than the initial long period. Both findings have been ascribed to the straininduced melting-recrystallization process as generally claimed in the literature. The microfibrils diameter, D f , was shown to depend on the draw temperature and initial microstructure in a different way as L pf . The evolution of D f was shown to correlate with the interfacial layer thickness that mainly depends on the chemical structure of the chains. It was concluded that, in contrast to L pf , the microfibril diameter should not be directly sensitive to the strain-induced melting-recrystallization. The proposed scenario is that after the generation of the protofibrils by fragmentation of the crystalline lamellae at yielding, the diameter of the microfibril during the course of their stabilization should be governed by the chain-unfolding and subsequent aggregation of the unfolded chains onto the lateral surface of the microfibrils. The morphogenesis of the microfibrils should therefore essentially depend on the chemical structure of the polymer that governs its crystallization ability, its chain topology and subsequently its fragmentation process at yielding. This scenario is summed up in a sketch.