In this study, rubber blend of natural rubber (NR) and styrene butadiene rubber (SBR) at 80/20 NR/SBR was reinforced with nanosilica (nSiO2) and polystyrene-encapsulated nanosilica (PS-nSiO2) in the latex state. The latex of PS-nSiO2 was synthesized by in situ differential microemulsion polymerization using sodium dodecyl sulfate and azobisisobutyronitrile as the surfactant and initiator, respectively. The nanoparticles at the amount of 0.1, 0.2, 0.3 and 0.4 parts per hundred of rubber (based on dry weight of nSiO2) were dispersed in the rubber blend compound and subsequently cured at 80°C for 3 h to prepare rubber nanocomposites. Using this technique, nanoparticles could be well dispersed in the rubber matrix. The influences of the nSiO2 and PS-nSiO2 on the mechanical and thermal properties of the resulting nanocomposites were quantified and compared. It is found that the tensile properties and thermal stability of the rubber blends were improved with the appropriate amounts of the nanofillers. However, the PS-nSiO2 exhibited reinforcing efficiency superior to nSiO2 with the same nSiO2 content due to the stronger rubber-filler interfacial adhesion.