Microencapsulation ammonium polyphosphate used as flame-retardant in polypropylene was prepared with hydroxyl silicone oil (HSO) and melamine-formaldehyde (MF) resin in this article. Fourier transform infrared and energy dispersive spectrometer were used to identify the structure of HSO-MFAPP. For the HSO-MFAPP/polypropylene (PP) composites, the flame retardant effect was evaluated by the limiting oxygen index and UL-94 testing, the compatibility was observed with scanning electron microscope, and the thermal stability was studied by thermogravimetric analysis. The results showed that the microencapsulation of ammonium polyphosphate (APP) with HSO-MF was prepared by in situ polymerization, and the flame retardant properties and water resistance of the PP/HSO-MFAPP/pentaerythritol (PER) composites were much better than the ones of the PP/APP/PER composites. Moreover, the compatibility of HSO-MFAPP with PP was better than that of unmodified APP. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012