Influence of silane coupling agent [bis-(3-(triethoxysilyl)-propyl)-tetrasulfide, TESPT] on bound rubber formation of carbon black-filled NR/SBR blend compounds was investigated. Bound rubber formation of carbon black-filled NR compounds with and without TESPT was also examined. Bound rubber contents of the NR/SBR blend compounds were less than those of the single NR compounds. Secondary layer bound rubber content of the NR/SBR compound containing TESPT was greater than that of the compound without TESPT, whereas formation of the primary layer bound rubber was not nearly affected by TESPT. For the single NR compounds, both the primary and secondary layer bound rubber contents of the compound containing TESPT were greater than those of the compound without TESPT. TESPT enhances the bound rubber formation by making sulfur crosslinks between NR chains in the compound. The secondary layer bound rubber contents of the NR/SBR blend compounds were lower than the primary layer ones, while for the single NR compounds the secondary layer bound rubber contents were greater than the primary layer ones. Rubber ratios of NR and SBR of bound rubbers of the NR/SBR blend compounds were determined using thermogravimetric analysis (TGA). The NR/SBR rubber ratio of the secondary layer bound rubber was much greater than that of the primary layer one.