The fatigue behavior of polypropylene/coir fiber composites was investigated. Composites were prepared according to an experimental statistical design, in which the independent variables coir fiber and compatibilizer content were varied. The compatibilizer used was maleic anhydride grafted polypropylene (PP-g-MA). Compatibilizer free composites were also prepared. Composites were prepared in a corotating twin-screw extruder and the mechanical behavior of polypropylene/coir fiber composites were assessed through monotonic (tensile) and cyclic (fatigue) tests. Fatigue load controlled tests were conducted under tension-tension loads at a frequency of 6 Hz. The fracture mechanism was accompanied by surface fracture analyses using scanning electron microscopy (SEM). The results indicated the need for using compatibilizer in the composites; however, increase in compatibilizer content did not affect composite fatigue lifetime. Coir content was the variable with the strongest effect on composite properties; increasing this variable caused significant increase in fatigue life. POLYM. ENG. SCI., 53:2159-2165, 2013. (c) 2013 Society of Plastics Engineers