Abstract: The present invention relates to a process for making a polymer masterbatch with silica nanoparticles highly uniformly dispersed therein by using a stable aqueous emulsion of compatibilized silica nanoparticles, the sizes of which are mechanically reduced to nanoscales. Adequate reduction of the silica particles to nanoscales is essential for the formation of a desired emulsion of highly stability. The said emulsion of compatibilized silica nanoparticle can be easily mixed under mild conditions with a polymer in latex form, which can be high purity natural rubber, synthetic rubber and/or a blend thereof. The bifunctional organosilane coupling agent bonded-Silica nanoparticles are subsequently incorporated into the rubber polymer network during coagulation with formic acid. The inventive rubber-silica masterbatch thus obtained is useful for manufacturing rubber and other polymer composite products.