Al(OH)(3)/polypropylene (PP) composites modified by in situ-functionalized polypropylene (FPP) were prepared by a one-step melt-extrusion process. The effect of in situ FPP on the crystallization and melting behavior, melt-flow index, limiting oxygen index, thermal degradation, mechanical properties, and fracture morphology of Al(OH)(3)/PP composites was studied. Formation of in situ FPP resulted in a decreased crystallization temperature and melting point of PP in the composites, an increased melt-flow index, and improved tensile and flexural strengths of Al(OH)(3)/PP composites, whereas the thermal degradation behavior and limiting oxygen index was not been influenced. The impact strength of the Al(OH)(3)/PP composites modified by in situ FPP depended upon the content of the initiator, dicumyl peroxide, and the monomer, acrylic acid. 2002 John Wiley Sons, Inc.