Kenaf fiber mats were modified with N-methylol acrylamide (NMA) solution with either sulfuric acid or p-toluenesulfonic acid (TsOH) as a catalyst. The (unmodified-kenaf)-UPE composites and (NMA-modified-kenaf)-UPE composites were obtained by a compression molding process. Orthogonal experiments were used for determination of optimum modification variables for maximizing the flexural strength of the resulting (NMA-modified-kenaf)-UPE composites. Three variables with three levels (NMA usage: 1, 3, and 5 wt %; usage of acid catalyst: 0.1, 0.2, and 0.3 wt %; and oven-dry temperature for NMA-treated kenaf fiber mats: 80, 103, and 125 degrees C) for each variable were used in the orthogonal experiments. Modifications of kenaf fibers with TsOH as a catalyst significantly increased the flexural strength. Modifications of kenaf fibers with either sulfuric acid or TsOH as a catalyst significantly enhanced the flexural modulus and significantly reduced the water-uptake rate of the resulting kenaf-UPE composites. The improved interfacial adhesion between kenaf fibers and UPE resin was confirmed by characterization of NMA-modified fibers with Fourier transform infrared spectroscopy, as well as scanning electron microscopy images. The mechanisms for the improved strengths and water-resistance by the NMA modifications of the kenaf fibers were discussed. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012