Rubber nanocomposite have been studied in many engineering fields. In this paper, to improve mechanical properties of the styrene butadiene rubber (SBR), different amounts of nano-calcium carbonate (nano-CaCO3) in combination with organoclay (OC) were employed in this rubber. Mixing of the rubber and nanoparticles was carried out in an internal mixer. Curing materials were also added to the compound in the internal mixer at a different temperature. The curing behavior, crosslink density and mechanical properties of the samples were evaluated and discussed in relation to the structure of nanocomposites. The obtained results revealed that the tensile strength of nanocomposites with both nanoparticles was higher than that of neat SBR and nanocomposites containing either of them. This was attributed to the effect of nano-CaCO3 on the dispersion and intercalation of the OC as well as the variation of crosslink density of the samples which was confirmed by the rheometry test. In addition, an increase in the elongation at break was observed in the samples with nano-CaCO3 in comparison to the corresponding samples without it.