Triethoxysilyl modified styrene-butadiene rubber latex was synthesized by the emulsion copolymerization of 2-(3-triethoxysilylpropyl)-1, 3-butadiene with styrene and 1, 3-butadiene. This latex was mixed with unmodified SBR latex and tetraethoxysilane to obtain SBR-silica composites by sol-gel reaction in the latex. Evaluation of mechanical properties demonstrated that tensile modulus and tensile strength, elongation at break improved with increasing of the amount of modified SBR. SEM observation for SBR-silica composites suggested that particle size of silica dispersed in unvulcanized rubber matrix grew larger with decreasing of silica content.