PP was endowed with excellent flame retardant properties by great mount of Mg(OH)(2) particulates. It was found that the mechanical properties of PP was severely deteriorated, caused by the bad compatibility between filler and matrix, as well as the agglomerate of Mg(OH), particles in PP matrix. The addition of rubbery phase as the third component could remarkably improve the Izod impact strength of PP highly filled inorganic flame retardant agent. In this article, EPDM and sulfated EPDM ionomer were adopted as the third component to improve the mechanical properties of PP/Mg(OH)(2) composites. The composites have better mechanical properties using sulfated EPDM ionomer as the third component than that of neat EPDM. The theoretical analysis and SEM photographs showed that even dispersion of particulates and the less domain size of rubbery phases as well as the better adhesion between rubbery phase and PP matrix were the main reasons for the improvement of the mechanical properties. (c) 2006 Wiley Periodicals, Inc.