Polypropylene (PP) is one of the most widely used materials because of its rational mechanical characteristics. However, the application of PP is significantly limited and infrequent due to its easy combustion and dripping properties. To improve the thermal versatilities of PP, halogen-containing compounds, alone or in conjunction with antimony trioxide, have frequently been used in the flame retarding PPs. Halogen-containing flame retarding agents, however, are supposed to produce corrosiveness, smoke emissions, and toxicity which are combustion products. In this study, red phosphorous, metal oxides, and zinc borate were used as flame retarding agents for PP. The thermal properties of injection-molded PP specimens were characterized by thermogravimetric analysis, differential scanning calorimetry analysis, and UL94 flame retardation tests. Results showed that increased char formation on the surface can confirm flame retardation of the UL94 V0 grade of PP.