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Abstract: Applications insulated metal substrates (IMS) for high-density and high-power mounting are greatly extending with miniaturizing of electronic components. Recently, aluminum nitride film has been used as a potential insulator and/or passivation material in insulated metal substrate because of its high intrinsic thermal conductivity, low thermal expansion coefficient, low dielectric constant and high electric resistivity. In this investigation, AlN films were deposited on Al substrates by radio frequency (RF) reactive magnetron sputtering. The metal-interfacial insulator layer-metal (Al/AlN/Al MIM) structures were obtained with AlN layer on Al substrates. Electrical properties of the MIM structures were investigated by meaning of C–V and C-f characteristics in the frequency range of 100Hz–500kHz and voltage range of −4 V to 4 V. Experimental results show that the dielectric constant of this structure decreases gradually with increasing frequency. While the dielectric loss tangent was tested from low frequency to high frequency, it is found that the dielectric loss tangent decreases from 0.03375, reaches a minimum (0.00424) at approximately 65kHz and then increases sharply. These results are in accordance with modified model of Goswami and Goswami for such structure. The dielectric dispersion is observed due to distribution of interface states as well as ionized space charge carriers such as the oxygen atoms, nitrogen vacancies and defects. The AC conductivity results show that the electrical resistance decreases as the frequency increasing due to hopping type conduction. [Copyright &y& Elsevier]