A phosphoric acid-pentaerythritol-melamine copolymer was selected as an intumescent flame retardant (IFR). The influence of a-methacrylic acid grafted polypropylene (PP-g-MAA) on the properties and compatibility of IFR/PP composites was studied. The results obtained from mechanical tests and scanning electron microscopy showed that PP-g-MAA was a true coupling agent for IFR/PP blends, but it did not change the necessary flame retardancy. The cocrystallization between bulk PP and PP segments of PP-g-MAA was proved by wide-angle X-ray diffraction analysis. A flow test showed that the flow behaviors of composites in the melt were those of a pseudoplastic liquid and it was very small for PP-g-MAA and affected the rheological behavior of the PP/IFR composite.