Styrene-butadiene rubber (SBR) and nitrile rubber (NBR) were reinforced with silica by the sol-gel method of tetraethoxysilane (TEOS) in latex. The sol-gel reaction proceeded in the mixture of latex, TEOS, water, and a catalyst. The compound that involved silica was crosslinked with the curing agent at 150 C. The amount of TEOS added and the molar ratio of TEOS to water ([H2O]/[TEOS]) in the mixture were related to the silica content in the compounds, particle size, and reinforcing behavior of silica. The particle size and reinforcing behavior of the silica were greatly influenced by the amount of TEOS and [H2O]/[TEOS]. The diameter of the dispersed silica particles in cured rubbers was controlled by the amount of TEOS and [H2O]/[TEOS], which were smaller than 100 nm. By the dispersion of the fine silica in the nanometer-order level, the tensile strength of the compound was drastically improved. The tensile strength of SBR was over 30 MPa, and of NBR, over 25 MPa. Compounds with a high tensile strength had a high bound rubber fraction and a very strong interaction of the fine silica with rubber.