The radio frequency permittivity and permeability spectra of Fe-Si-Ni/epoxy magnetic composite materials were investigated in detail. A relaxation frequency dispersion of permittivity has been observed for the composites below percolation threshold, and tunable negative permittivity due to the plasma oscillation of delocalized electrons in interconnected Fe-Si-Ni has been obtained in the composites above but still near percolation threshold. The realization of negative permittivity in Fe-Si-Ni/epoxy composites can be applied for the electromagnetic shielding materials. (C) 2015 Elsevier B.V. All rights reserved.