In this study, we introduced the concept of in-situ nanofibrillation as an efficient, low-cost, and environmentally friendly technique for the enhancement of polycarbonate (PC) mechanical and rheological properties. PC/Polyamide 6T (PA6T)-fibril composites are prepared by a twin-screw extruder and afterwards, elongated by melt spinning and spunbond systems. Modifications of the mechanical and rheological properties of PC via fiber-spinning/spunbonding of PC/PA6T are achieved by elongation of the second phase (PA6T) properly in the main matrix (PC). Morphological observations demonstrated the well dispersed and distributed fibrillar phase of PA6T with high aspect ratios in the PC matrix. DSC results of the spherical blends and fibers of PC/PA6T showed the compatibility of these two polymers, which has emerged as a convenient route to tune the viscoelastic, and consequently improvement the properties of the matrix. Rheological investigations proved that PC with nanofibrillated PA6T has dramatically improved melt elasticity compared with neat PC due to presence of fibrillar network in the matrix. Uniaxial extensional viscosity measurements showed a strain-hardening behavior in the fiber-spun PC/PA6T, which was not observed in the neat PC or in the melt-blended PC/PA6T. PC with nanofibrillated PA6T also improved the mechanical properties, especially the ductility, the toughness and the impact strength, while increasing the stiffness, in comparison with the virgin PC. The change in the tensile properties was governed by the aspect ratios of the fibrils. Hence, having highly stretched fibrils with the fibrillar network is an effective way to enhance the mechanical and rheological properties.