Polyurethane (PU) composites reinforced with graphene nanosheets (GNSs) were prepared by in situpolymerization. Graphene nanosheets, which were derived from chemically reduced graphite oxide (GO) nanosheets, were characterized by solid-state ¹³C MAS NMR, XRD and FTIR. A morphological study showed that, due to the formation of chemical bonds, the GNS was dispersed well in the PU matrix. With the incorporation of 2.0 wt% of GNSs, the tensile strength and storage modulus of the PU increased by 239% and 202%, respectively. The nanocomposites displayed high electrical conductivity, and good thermal stability of PU was also achieved. The facile and rapid technique presented here will provide an effective and promising method of preparing graphene-based polymer composites.