This paper Studied the permittivity of carbon black/epoxy composites at microwave frequencies. The measurements were performed for permittivity at a frequency band of 0.5-1.8 GHz and for DC conductivity to find the percolation threshold of composite samples of 0.0-1.5 wt %. of carbon black in epoxy resin. The experimental results show that the complex permittivity of the composite depends strongly on the nature and concentration of the carbon black dispersion. The frequency spectrums of dielectric constants and AC conductivities Of Composites were investigated by using the descriptions found in percolation theory. A new model, that is a branch of Lichtenecker-Rother formula, is proposed to obtain a rule of mixture to describe the complex permittivity of the composite as function Of frequency and concentration of carbon black. (c) 2006 Wiley Periodicals, Inc.