Silica particles were prepared in the SBR latex by sol-gel method under various reaction conditions. The relationship between the reinforcing behavior of silica and the formation of bound rubber structure in composites was investigated. Measurements of the fraction of bound rubber in composites suggested that bound rubber in composites, which had high tensile strength, were largely increased during the compounding of vulcanization reagents. On the other hand, bound rubber in composites, which had low tensile strength, were slightly changed during the compounding. SEM observation results for unvulcanizated composites suggested that silica particles breakdown from agromerates (1-10μm) to aggregates (10-1000nm) during the compounding of vulcanization reagents. It was also suggested that extreme size change of silica particles was occurred in case of bound rubber largely increased during the compounding. It was considered that reinforcing behavior and particle size of silica were closely related to increasing of bound rubber during the compounding.