The melamine phosphate (MP) was synthesized from melamine and phosphate acid. Aluminum oxide (Al2O3) was added into the MP during its synthesis. Fourier transform infrared spectroscopy (FTIR) was used to characterize the reaction products. The composites were prepared from polypropylene (PP) and intumescent flame retardant (IFR) additives of MP and pentaerythritol (PER). The effect of Al2O3 content on the properties of the composites was investigated by the limiting oxygen index (LOI), thermogravimetry (TG) and scanning electron microscopy (SEM). The results indicated that the LOI value of the composites could be enhanced obviously with suitable content of Al2O3. When the Al2O3 content was 6 wt% in MP, the LOI value increased to 35.5% from 31.0% for the composite without Al2O3, the amount of residual char at 600°C increased from 8.9 wt% to 13.9 wt%.