Ammonium 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. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 834-840, 2010