The melamine phosphate (MP) was synthesized from melamine and phosphate acid. Zinc oxide (ZnO) was used as a synergistic agent by adding into the MP during the synthesis process. The reaction products were characterized by Fourier transform infrared spectroscopy (FTIR). The properties of polypropylene (PP) composites containing intumescent flame retardant (IFR) additives of melamine phosphate and pentaerythritol (PER) were investigated by the limiting oxygen index (LOI), UL-94 vertical burning test, thermogravimetric analysis (TG) and scanning electron microscopy (SEM). The results indicated that the LOI value of the composites could be enhanced obviously with suitable content of ZnO. When the ZnO content was 6 wt% in MP, the LOI value reached 34.1%, which showed 10% higher than that of the composites without ZnO.