The effect of alkyl silicone oil (ASO) as a compatibilizer on the compatibility of polypropylene (PP)/microencapsulated ammonium polyphosphate (MAPP) composites was investigated. The data from the mechanical tests showed that when 0.25% ASO was added, the tensile strength of the PP/MAPP/ASO composites increased approximately 41% compared with that of the PP/MAPP composite. The scanning electron microscopy results illustrate that ASO improved the interfacial adhesion between MAPP and PP. Polarized optical microscopy and X-ray diffraction demonstrated that ASO decreased the agglomeration of MAPP and made MAPP disperse homogeneously in PP. Flow tests and IR spectroscopy indicated that ASO played the role of lubricant in PP/MAPP, and the flow behavior of composites in the melt was that of a pseudoplastic liquid, in which MAPP interacted with ASO by molecular interaction. All of the investigations showed that a suitable amount of ASO improved the compatibility of the PP/MAPP composite. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1176-1182, 2011