Natural-rubber-based hybrid composites were prepared by the mixture of short cellulose fibers and silica of different relative contents with a 20-phr filler loading with a laboratory two-roll mill. The processability and tensile properties of the hybrid composites were analyzed. The tensile modulus improved, but the tensile strength and elongation at break decreased with increasing cellulose fiber content. The scorch safety improved with the addition of 5-phr cellulose fiber in the composites. The Mooney viscosity significantly decreased with increasing cellulose fiber content. To modify the surface properties of the cellulose fiber and silica fillers, a silane coupling agent [bis(triethoxysilylpropyl) tetrasulfide, or Si69] was used. The effects of Si69 treatment on the processing and tensile properties of the hybrid composites were assessed. We found that the silane treatment of both fillers had significant benefits on the processability but little benefit on the rubber reinforcement. The strength of the treated hybrid composite was comparable to that of silica-reinforced natural rubber. Furthermore, to investigate the filler surface modification and to determine the mixing effects, infrared spectroscopic and various microscopic techniques, respectively, were used. From these results, we concluded that the fillers were better dispersed in the composites, and the compatibility of the fillers and natural rubber increased with silane treatment. In conclusion, the hybridized use of short cellulose fibers from a renewable resource and silica with Si69 presented in this article offers practical benefits for the production of rubber-based composites having greater processability and more environmental compatibility than conventional silica-filler-reinforced rubber. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 120: 3242-3254, 2011