Effects of silica on the vulcanization reaction by sulfur/N-cyclohexyl-2-benzothiazolylsulfenamide (CBS) in isoprene rubber were studied by FT-IR and ESR. It was found that the induction time of vulcanization reaction was shortened by the addition of silica. However, the efficiency of vulcanization at the final stage of reaction was lower for silica filled sample than for unfilled one. The structure of CBS in the vulcanization system was investigated by the FTIR at a crosslinking temperature. It was found that by the addition of silica in the system new peaks appeared at 1541 and 1598cm^-1,suggesting a decomposition of CBS by silica. Further, the concentration of sulfur radical during the vulcanization was decreased by the addition of silica. However, the decomposition was suppressed by the removal of water molecules absorbed directly on silica surface. These results indicated that at an initial stage of vulcanization, most of CBS was decomposed by silanol groups which were covered with the adsorbed water, leading to the fast vulcanization reaction by sulfur, however, the concentration of CBS dropped quickly at the initial stage of vulcanization, resulting in the low efficiency of vulcanization. These results were explained by the decomposition of CBS by acid catalyst ability of silanol groups.