Kenaf fiber (KF) reinforced recycled polypropylene (RPP) composites were produced by melt cast method. To improve interfacial adhesion between fiber and RPP matrix, fiber surface modification was carried out by means of ultrasound treatment. Maleic anhydride grafted polypropylene (MAPP) was used as a coupling agent. Composites were examined by mechanical test, melt flow indexing test, scanning electron microscopy, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Water uptake analysis and accelerated weathering test were carried out to find the suitability of the composites in outdoor application. Among the raw fiber contents ranging 1050 wt % in the composites, the maximum tensile strength (TS) was observed at 40 wt % KF loading without using MAPP. Treated KF-based composite with MAPP promotes this maximum TS value, which is 57% higher than that of raw KF-based composite. TGA and DSC analyses exhibit an enhancement of thermal stability in treated KF-reinforced RPP composites with MAPP. Incorporation of MAPP in the composites shows higher activation energy, suggesting improved interfacial bonding between fibers and matrix. Response surface method was employed to demonstrate the optimal treatment parameters for TS, showing excellent agreement with the observed values. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013