The addition of methacrylate-functional polyhedral oligomeric silsesquioxane (MA-POSS) nanoparticles to styrene-butadiene rubber (SBR) composites was evaluated in terms of rheological, mechanical, and morphological properties. The aging process and cross-link density of rubber nanocomposites were examined in presence of MA-POSS. The MA-POSS could directly participate in the vulcanization reaction due to its methacrylate groups and its concentration affected the rheological and mechanical properties of rubber. The sulfur vulcanization characteristics of MA-POSS/SBR nanocomposites were determined by moving die rheometer. It was indicated that the addition of MA-POSS to SBR composites retarded the curing time and decreased the cross-link density. Due to an effective interaction between MA-POSS nanofiller and SBR matrix and excellent dispersion of POSS, the mechanical properties of nanocomposites were higher than those of the reference compound without MA-POSS. However, there was no significant change investigated in tensile retention in presence of MA-POSS. The presence and the random distribution of MA-POSS in SBR matrix was confirmed by TEM analysis. The cross-link densities of the composites, which were calculated using Flory-Rehner equation, decreased with increasing amount of POSS nanofiller. It was finally revealed that the thermal aging dramatically reduced the mechanical properties and cross-link density of MA-POSS/SBR nanocomposites.