A novel flame-retardant system of isotactic polypropylene (iPP) is prepared using polyethylene glycol (PEG) and ammonium polyphosphate (APP). The flammability of iPP/PEG/APP composites containing 20 wt% APP improves with the increase of PEG concentration in the range of 315 wt%. The limit oxygen index (LOI) of iPP/PEG/APP composites reaches up to 30% with 15 wt% PEG concentration in the composites. At the same time, the mechanical properties of iPP/PEG/APP composites demonstrate that PEG can enhance toughness of iPP/APP composites. The results of cone calorimetry prove the synergistic effects of PEG and APP on intumescent flame-retardant iPP, and those of the thermogravimetric analysis(TGA) reveal that iPP/PEG/APP samples decompose faster than iPP/APP composites. Investigated by scanning electronic microscopy (SEM), the morphology and structures of residues generated during LOI tests confirm the formation of effective char layer, and that the improvement of the flame retardancy and the impact strength of the composites are thanks to the presence of PEG. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers