Dynamic mechanical properties of high-cis butadiene rubber were investigated, The butadiene was grafted with alkoxysilane monomer and reinforced with in situ-formed silica by the sol-gel process, Tan delta at room temperature was not increased with growing silica content. A homogeneous dispersion of very small silica particles was found by TEM observation, These results suggest that well-dispersed silica particles are effective not only as reinforcing fillers, but also as crosslinking points which give rise to an elastomeric network structure of low dynamic loss.