In this study, we introduce the concept of in-situ nanofibrillation as an efficient, low-cost, and environmentally friendly technique for the enhancement of polycarbonate (PC) properties. PC/polyamide 6T (PA6T)-fibril composites are prepared by a twin-screw extruder. Modifications of the mechanical and rheological properties of PC via fiber-spinning of PC/PA6T are distinguished by elongation of the second phase (PA6T) properly in the main matrix (PC). Morphological observations showed the well dispersed fibrillar phase of PA6T with a high aspect ratio in the PC matrix. Rheological investigations proved that PC with nanofibrillated PA6T has dramatically improved melt elasticity compared with neat PC. 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 neat PC. The change in the tensile properties was governed by the draw ratio. Hence, having stretched fibrils is an effective way to enhance the mechanical and rheological properties. 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.