The voltage (V) and frequency (f) dependence of dielectric characteristics such as dielectric constant (epsilon'), dielectric loss (epsilon ''), dielectric loss tangent (tan (5) and real and imaginary part of electrical modulus (M' and M '') of the (Ni/Au)/GaN/Al0.3Ga0.7N heterostructures have been investigated by using experimental admittance spectroscopy (capacitance-voltage (C-V) and conductance-voltage (G/w-V)) measurements at room temperature. Experimental results show that the values of the epsilon', epsilon '', tan delta and the real and imaginary parts of the electric modulus (M' and M '') obtained from the C and G/w measurements were found to be strong function of frequency and applied bias voltage especially in depletion region at low frequencies. These changes in dielectric parameters can be attributed to the interfacial GaN cap layer, interface polarization and a continuous density distribution of interface states and their relaxation time at metal/semiconductor interface. While the values of the epsilon' decrease with increasing frequencies, tan delta, M' and M '' increase with the increasing frequency. Also, the dielectric loss (epsilon '') have a local maximum at about frequency of 100 kHz. It can be concluded that the interface polarization can occur more easily at low frequencies with the number of interface states located at the metal/semiconductor interface. (C) 2010 Elsevier Ltd. All rights reserved.