Rubber reinforcement using in situ prepared silica by sol-gel method using tetraethoxysilane (TEOS) was carried out on styrene-butadiene rubber (SBR) latex. SBR latex, TEOS, water and catalyst were mixed at room temperature. After keeping the temperature of the mixture at 40°C for 24hr to proceed sol-gel reaction, the system was held at 80°C for 8hr under vacuum to remove TEOS, water and alcohol which were formed by the sol-gel reaction. Vulcanization of the SBR/silica stock was conducted at 150°C by hot-press. Silica content of composite was determined by thermogravimetry (TG). Tensile strength, 300% tensile modulus and elongation of vulcanizates were measured by a tensile tester. Silica particles dispersed in SBR matrix were observed by transmission electron microscope (TEM). Silica content of composite could be controlled by amounts of added TEOS to SBR latex. Mechanical properties were improved by in situ prepared silica, compared with the gum stock of SBR. In addition, in situ prepared silica showed high reinforcement effect than white carbon that was compounded by the conventional method. Particle size of in situ prepared silica dispersed in the SBR matrix was 0.1-0.5μm. Molar ratio of TEOS to water added to SBR latex influenced to mechanical properties of the composite.