Coir fiber-reinforced polypropylene composites (PP/CF), compatibilized with maleic anhydride grafted polypropylene (PPMAH) and stabilized with a hindered amine, HALS (Tinuvin (R) 791) and UV absorber (Hostavin (R) ARO 8), were prepared in a co-rotating twin-screw extruder. The effects of additives and their interactions were investigated by analyzing the mechanical properties, in tensile strength, scanning electron microscopy (SEM) and Differential Scanning Calorimetry (DSC). The results indicate that the incorporation of coir, in the presence of the compatibilizer, significantly increases tensile strength, but in the absence of the compatibilizer, the natural fibers act as stress concentrators, allowing a lower tensile strength to the sample. The joint presence of Tinuvin (R) and the compatibilizer has a negative effect on tensile strength and on the compatibility of the natural fiber with the matrix. UV absorber does not affect the compatibility of the PP/CF composites. (C) 2017 Society of Plastics Engineers