In this work, nanocomposites based on natural rubber (NR) as well as chlorobutyl rubber (CIIR) containing different amounts of graphene nanoparticles (GNP) were prepared by melt mixing method. The dispersion of GNP in each of the two elastomers was examined by X-ray diffraction analysis (XRD) and field emission scanning electron microscopy (FE-SEM). The obtained results showed a better dispersion of GNP in CIIR. However, the chemical interaction between NR and GNP was confirmed by Fourier transform infrared (FTIR) spectroscopy. The use of GNP and increasing its value caused an increase in torque difference in the rheometric test, which was attributed to the increase in the crosslink density and the swelling test confirmed it. The results of static and dynamic mechanical tests showed the reinforcing effect of GNP on increasing the tensile strength and elastic modulus of NR and CIIR. The oxygen gas permeability of NR and CIIR, in the presence of 1.5 phr GNP, decreased by 26% and 45%, respectively. By using 1.5 phr GNP, the temperature of the onset of degradation and the temperature of the maximum rate of degradation decreased slightly in NR and increased in CIIR. According to the obtained results, it can be stated that the presence of graphene in CIIR has a greater effect on improving mechanical and barrier properties than in NR. Also, the melt mixing method used in this work has led to a greater improvement in the properties of these rubbers compared to the solution method reported in the literature.