The effects of filler loading and a new silane coupling agent 3-octanoyl-thio-1- propyltriethoxysilane (NXT silane) on the polymer-filler interaction and mechanical properties of silica-filled and carbon black-filled natural rubber (NR) compounds were studied. Silica (high dispersion silica7000GR, VN2, and VN3) and carbon black (N330) were used as the fillers, and the loading range was from 0 to 50 phr. The loading of NXT silane was from 0 to 6 phr. Experimental results show that the maximum and minimum torques of silica and carbon black-filled NR increase with increasing filler loading. With increasing filler loading, the scorch time and optimum cure time decrease for carbon black-filled NR, but increase for silica-filled NR. The minimum torque, scorch time, and optimum cure time decrease because of the presence of NXT silane. For the carbon black and silica-filled NR, the tensile strength and elongation at break have maximum values, but the hardness, M300, M100, and tear strength keep increasing with filler loading. The mechanical properties of silica-filled NR were improved in the presence of NXT silane. With increasing filler loading, the storage modulus of filled NR increases, but the loss factor decreases. Carbon black shows the strongest polymer-filler interaction, followed by VN3, 7000GR, and VN2. (C) 2005 Wiley Periodicals, Inc.