This paper studies the dielectric properties of an elastomer filled with spherical Ni particles of different sizes the range 35–115 µm. The experimental analysis is followed by an examination of the permittivity/conductivity mixing rules. A new mixing formula for the composite system consisting of spherical metallic particles and the polymer matrix is presented. This formula includes both Ni particle content and Ni particle size as two independent variables. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.