To control the interfacial interaction in magnetic metal nanoparticles-filled polymer composites, surfaces of iron, cobalt, and nickel nanoparticles were grafted by irradiation-induced polymerization. On the basis of the study of dynamical mechanical behavior, thermal stability, and magnetic performance of the composites prepared by either solution mixing or in situ polymerization, the structure-property relationships of the composites are a function of interfacial interaction and the dispersion state of the nanoparticles. In addition, grafting of polymers onto the surface of the metal nanoparticles changed the surface magnetic state, leading to the possibility of purposely tailoring the magnetic behavior of the composites. (C) 2003 Wiley Periodicals, Inc.