Interface effect was a key mechanism for the deformation of metal matrix composites, which has not been well understood yet in those reinforced with nanoreinforcements. In this study, we prepared graphene nanosheet reinforced aluminum (GNS/Al) composites exhibiting improved tensile strength from 233 to 287 MPa while retaining good uniform elongation from 5.5 to 5.8% compared with the unreinforced Al, thanks to the interface-induced strain hardening capability. The strain hardening behaviors of the GNS/Al composites were discussed in terms of forest hardening and back stress hardening by using the tensile loading-unloading tests and further quantitatively analyzed through a modified strain hardening model based on the dislocation behaviors, where the contributions of grain boundaries and interfaces were distinguished. It turns out that the interface-induced forest hardening was the main reason for improving the strain hardening capability and uniform elongation of the GNS/Al composites. Microstructural characterization revealed that, the additional strain hardening capability of the composites should be from the higher dislocation storage capability of the GNS-Al interfaces than Al grain boundaries and the accommodation of geometrically necessary dislocations near the GNS-Al interfaces. The present work provides a new insight to the design of both strong and ductile metal matrix nanocomposites.