Composites based on polypropylene and glass fiber were prepared by melt mixing. The effect of magnesium hydroxide as a flame retardant on flammability and thermal behavior of glass fiber/polypropylene composites was studied. Ratio of glass fiber to magnesium hydroxide in each composite sample was varied. Maleic anhydride grafted polypropylene (MAPP) was used to improve the interfacial adhesion between polypropylene and fillers. Flammability and thermal behavior of the composites were examined using a horizontal burning test and a thermogravimetric analyzer, respectively. Morphology and mechanical properties of the composites were also investigated. Magnesium hydroxide reduced the flammability while improved thermal decomposition temperature of the polypropylene composites. However, magnesium hydroxide showed a negative impact on the tensile strength of the polypropylene composites.