Interfacial properties of fiber reinforced polyamide 6 (PA6) composites are important to their mechanical behaviors. In this work, polyethyleneimine (PEI) functionalized carboxylic multi-walled carbon nanotubes (PEI-CNT) were grafted onto the surface of glass fibers (PEI-CNT-GF) to reinforce PA6 by improving interfacial properties. The PEI-CNT containing plenty of active groups formed a compact and homogeneous network structure on the fiber surface. The network structure and high surface reactivity were beneficial to better interfacial performance which improved the mechanical properties of glass fibers reinforced PA6 composite. Results of DSC and POM illustrated that the PEI-CNT-GF could induce the PA6 transcrystallization (TC) along the fiber surface, which strengthened the physical connection at the interface. The tensile and flexural strength of PA6/PEI-CNT-GF composite were about 7% higher than those of PA6/raw GF. This study provides a feasible method for interfacial modification in glass fibers reinforced PA6 composites and it may be applicable to other thermoplastic composite systems.