The mechanical properties of natural rubber reinforced with exfoliated graphene oxide (GO) nanoplatelets have been studied to evaluate the reinforcement efficiency of the filler. Tensile tests on rectangular specimens have been carried out at different strain rates for deformations up to 600%. Tensile tests showed a significant increase in the Young's modulus of the composites even at low filler loading (+50% at 1.00. wt%) that corroborates the strong interfacial interaction between GO and natural rubber. The viscoelastic properties are investigated through relaxation tests and loading-unloading cycles at different strain rates. It is shown that the Young's modulus increases with the strain rate and the dissipation increases (+250% at 0.75. wt%) for increased GO content because of the increased friction caused by the presence of high aspect-ratio GO platelets. These findings indicate significant improvements in the mechanical properties at much lower filler concentration than those usually achieved with carbon black or carbon nanotubes.