Abstract: The surface of coir fiber was treated with different concentrations of alkali solution, coupling agent or their mixture, and then the coir-fiber-reinforced polypropylene (PP) composites were prepared by a compression-molding technology. The effect of surface modification on the surface characteristics of coir fiber and the mechanical properties of composites were investigated. The results indicated that the combination of alkali and coupling agent was an optimum modification way to improve the mechanical properties of the composites. The tensile strength of the composites increased with the concentration of coupling agent at the same alkali concentration. The modification of coupling agent generated a more significant effect on the mechanical properties than that of alkali. Moreover, the modification of coupling agent also influenced the flexural strength of the composites remarkably, and the mechanical properties were found to increase linearly with the concentration of coupling agent.