AbstractAmmonium polyphosphate (APP)/montmorillonite (MMT) nanocompounds were prepared. The crystal forms and morphologies of the nanocompounds were studied by XRD, FTIR, SEM, and TEM. The APP/MMT nanocompounds were applied to intumescent flame‐retarded polypropylene (PP) composites. The PP composites were studied by using oxygen index measurements, UL‐94 flame testing, thermogravimetric analysis, and mechanical measurement. It was found that the APP/MMT nanocompounds enhanced the flame retardancy of the IFR/PP compared with the form II APP and its mixture with micro‐MMT. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010