In this study, an N-alkoxy-hindered amine-based UV stabilizing agent (NOR-116) and nanosized silica particles (Aerosil R-972) were combined with a cyclic phosphonate based-flame retardant (FR; PCO-900) and incorporated into polypropylene via melt extrusion in a microcompounder. In order to stimulate the conditions in the favor of further processing such as fiber spinning, the content of additives in polypropylene was kept low (up to 6.5 wt %). The effects of the PCO-900, alone and in combination with NOR-116 and Aerosil-R972, on the flammability and thermal stability of polypropylene were evaluated by limit oxygen index (LOI) tests, cone calorimetry, and thermogravimetric analysis. The proposed system with 3.5 wt % PCO-900/1.5 wt % NOR-116/1 wt % Aerosil-R972 decreased the heat release, increased the LOI and thermo-oxidative stability, and, thus, improved the fire resistance of polypropylene. The possible mode of FR activity was also discussed based on the analysis. (c) 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48308.