Microfibers can be obtained by bicomponent spinning, followed by subsequent mechanical splitting. During process, two materials are coextruded in a die to form a unique complex morphology. Many factors affect these morphologies: melt viscosity and difference of crystallization temperature combined with polymers position. Consequently, fiber splitting can be improved by choosing an association of polymers with a stable interface and a poor adhesion. The aim of this study is to understand which intrinsic parameters of polymers allow to enhance bicomponent fiber's splitting. Bicomponent fibers (side-by-side and sheath/core) have been made with two grades of polypropylene and polyamide 6. Instable interface happens when a low-viscosity polymer flows around and encapsulates a high-viscosity material. Possible mechanism responsible of interface deformation is variation of shear rates through the morphology (highest shear rate is at the fiber periphery). DMA analysis reveals that fiber with polyamide as core exceeds the strength of fiber with polyamide as sheath. This increase of strength can be attributed to a better adhesion than fibers with PA6 in sheath. From experimental results, the position combined with the difference crystallization temperature shows poor or strong interface.