Bis(3-triethoxysilylpropyl) tetrasulfide (TESPT) was used to improve the interfacial adhesion between cotton fiber and natural rubber (NR). The crosslink density, interfacial adhesion, mechanical properties, dynamic mechanical properties, and morphology of NR/cotton fiber composites were investigated. The composites with TESPT had higher crosslink density, better mechanical properties, higher initial modulus, and higher yield strength than the composites without TESPT because of the difference in interfacial adhesion. The results of an interfacial adhesion evaluation, the high storage modulus and low damping values of the composites with TESPT, and the coarse surfaces of the pullout fibers implied the enhancement of interfacial adhesion. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 437-443,2009