In this study, a new and facile route has been developed to prepare graphene oxide (GO) reinforced polyamide 6 (PA6) composites and synthesize simultaneously PA6 grafted GO hybrid materials: ε-caprolactam (CL) was firstly fixed onto the GO sheets coupling by 4,4′-methylenebis(phenyl isocyanate), and then PA6 was grafted from the GO surface by in situ anionic ring-opening polymerization. The polymerization processing was effectively carried out at relatively low reaction temperature (150 °C) and in a short reaction time (20 to 40 min) by using a caprolactam magnesium bromide (C1) initiator in combination with a difunctional hexamethylene-1,6-dicarbamoylcaprolactam (C20) activator. The PA6 grafted graphene oxide (g-GO) was verified by 1H NMR, FTIR, TGA, XPS and AFM. The PA6 grafted GO sheets (g-GO) contain about 74 wt% polymers, which make the GO sheets homogenously dispersed in matrix and gain good interfacial adhesion. The tensile results show that the tensile strength and Young's modulus of the nanocomposites can be obviously improved by incorporation of g-GO at low contents. Furthermore, the crystallization temperature and degree of crystallinity of PA6-GO nanocomposites both increased in the non-isothermal crystallization process, especially for the composites with GO loading less than 0.2 wt%. This simple and effective approach is believed to offer possibilities for broadening the graphene applications with the development of PA6-graphene nanocomposites.