Ammonium polyphosphate (APP) and zinc borate (Zb), as flame retardants were incorporated into sisal fiber/polypropylene (PP) composites. Ratios of APP to Zb were varied. Maleic anhydride grafted polypropylene (MAPP) was used to improve the interfacial adhesion between polypropylene and fillers. Flammability of the composites was examined using a horizontal burning test, a vertical burning test, and a limiting oxygen index test. Morphology and mechanical properties of the composites were also investigated. The composite containing 30 phr of APP and 10 phr of Zb exhibited better flame retardancy than the composites containing other ratios of APP to Zb. The combination of APP and Zb insignificantly changed mechanical properties of the composites. In addition, SEM micrographs of the composites revealed good distribution of flame retardants and sisal fiber in PP matrix.