In this study, the kind of halogen-free flame-retardant containing phosphorus and nitrogen was blended with polypropylene (PP) by hot melt to improve the flame-retardant property of the composites. The cone calorimeter was used to investigate flame-retardant property of the composites. The formation mechanism of the char obtained from the combustion under air atmosphere at 700 degrees was studied by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Results demonstrated that the PP composite containing nitrogen and phosphorus showed good flame retardancy from cone calorimeter. According to XPS results, it had been found that the intumescent char produces phosphorus and polyphosphorus acid during drastic decomposition, which act as dehydration agents to drive the formation of the heat-resistant carbonaceous char by carbonization. There are also some glass materials, such as P2O5 compound, which can benefit the flame retardance. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013