Kenaf fibers are modified with three novel coupling agents that consist of an isocyanate and acrylated epoxidized soybean oil. The untreated-kenaf-UPE (unsaturated polyester) composites and coupling-agent-treated-kenaf-UPE composites are obtained by a compression molding process. Modifications of kenaf fibers with coupling agents significantly increase mechanical properties, and enhance water resistance of the resulting kenaf-UPE composites. Characterization of coupling-agent-treated-fibers with Fourier transform infrared spectroscopy reveals that coupling agents are covalently bonded onto kenaf fibers. Improved interfacial adhesion between kenaf fibers and UPE resin is confirmed with scanning electron microscopy. The mechanisms on which the chemical treatments of kenaf fibers improve the mechanical properties and water resistance are discussed. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128:1101-1109, 2013