In this work, we studied the mechanical and thermal properties of natural rubber (NR) based nanocomposites modified with graphene (Gr) and reduced graphene oxide (RGO). Gr nanoplatelets were modified using improved hummer’s method to functionalize their surface. The effect of Gr modification on thermal conductivity and mechanical properties of the prepared NR nanocomposites was carefully evaluated. It was observed that rubber nanocomposites modified with RGO exhibited higher thermal conductivity compared to those modified with pristine graphene. Also, it was demonstrated that using RGO was useful for NR composites preparation. Tensile characterization showed that the addition of 2 phr (phr: parts per hundred of rubber) of neat GR led to an increase of the tensile modulus at 100%, 200%, and 300% elongation by 15%, 12.5% and 8%, respectively. However, only 0.5 phr of RGO were needed to improve the tensile modulus by 15% at the above elongation points. This is due to the fact that graphene oxidation enhanced its dispersion inside the rubber composite and improved its mechanical properties.