Due to environmental reasons and their acceptable mechanical properties, natural fiber based polymer composites have been increasingly used in various applications, e.g. housing materials, automotive parts. However, their low thermal resistance and fire behavior limit the use of these materials. In this work, magnesium hydroxide (Mg(OH)2), as a flame retardant, was incorporated into sisal/polypropylene composites. Ratio of sisal fiber to Mg(OH)2 in each composite sample was varied. Maleic anhydride grafted polypropylene (MAPP) was also used to improve the interface of polypropylene and fillers. Flammability and thermal behavior of the composites were examined using a horizontal burning test and thermo gravimetric analyzer, respectively. Morphology and mechanical properties of the composites were also investigated.