In this article, the microencapsulated ammonium polyphosphate crystalline with form I (APP-I) coated with melamine-formaldehyde (MF) was prepared by in situ polymerization. Results of Fourier transform infrared spectra (FTIR), thermogravimetry (TG) energy dispersive spectroscopy (EDS), and scanning electron microscopy (SEM) demonstrate that APP-I is successfully microencapsulated with MF. Compared with APP-I, the microencapsulated APP-I with MF (MFAPP-I) is of much smaller spheroidal particle size and lower solubility in water. In this study, the polypropylene (PP)/APP-I/penpaerythritol (PER) and PP/MFAPP-I/PER composites are prepared, and flame retardancy, thermal stability, and microstructure of corresponding composites are carefully investigated by limiting oxygen index (LOI), UL-94 testing, TG, EDS, and SEM. Experimental results show that PP/MFAPP-I/PER composites have advantages over PP/APP-I/PER composites in terms of flame retardant properties and water resistance. Results of TG, SEM, and EDS show that the microencapsulated APP-I with MF resin is conducive to increase the amount of residual yield and improve thermal stability of PP/MFAPP-I/PER composites and the compatibility and dispersion of MFAPP-I. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013