The microstructure and mechanisms of reinforcement have been investigated in nanocomposites consisting of graphene nanoplatelets (GNPs) in natural rubber (NR). Nanocomposites with four different loadings of three different sized GNPs were prepared and were bench-marked against nanocomposites loaded with N330 carbon black. The microstructure of the nanocomposites was characterised through a combination of scanning electron microscopy, polarised Raman spectroscopy and X-ray computed tomography (CT), where it was shown that the GNPs were well dispersed with a preferred orientation parallel to the surface of the nanocomposite sheets. The mechanical properties of the nanocomposites were evaluated using tensile testing, and it was shown that, for a given loading, there was a three times greater increase in stiffness for the GNPs than for the carbon black. Stress transfer from the NR to the GNPs was evaluated from stress-induced Raman bands shifts indicating that the effective Young's modulus of the GNPs in the NR was of the order of 100 MPa, similar to the value evaluated using the rule of mixtures from the stress-strain data.