AbstractEffects of alkali treatment of rice straw flour on the mechanical properties of rice straw flour-polypropylene composites were investigated. Rice straw flour (40 mesh) was first treated with sodium hydroxide using two concentrations of sodium hydroxide, 5 and 10% (W/W), and two treatment times (45 and 90 min) for a total of four treatments. The composites were then made with the rice straw flour as a filler (30%), polypropylene (65%) as a matrix, and maleic anhydride (5%) as a coupling agent. The polypropylene/rice straw flour mixtures were blended in an internal Haake mixer and made into molds that were later used for mechanical testing. The results showed that the treatment of rice straw flour with 5% alkali (W/W) increased the tensile modulus and impact strength. Longer treatment time also resulted in a higher tensile modulus and impact strength. The fiber/matrix interaction was analyzed from the mechanical data and morphological (SEM) studies. Treatment of rice straw flour with 10% alkali (W/W), however, decreased these properties even under a longer treatment time. Increasing the alkali concentration and treatment time increased the flexural modulus, flexural strength, and tensile strength of the composites. The SEM results showed greater adhesion between the rice straw flour and the polypropylene matrix at higher alkali concentrations and longer treatment times.