Lithium niobate LiNbO3 was prepared as a thin film layered structure deposited on stainless steel substrate using e-gun evaporation. The Li doping was provided for by the formation of Li-Nb-O/Li/LiNb-O sandwich structure and annealing at about 250 °C. AC impedance spectroscopy measurements were performed on the samples at temperatures from the interval between 28 and 165 °C and in a frequency range of 10 -3 to 10 6 Hz. Using the values Z´ and Z´´ at different frequencies, the dielectric parameters -parts of the complex permittivity ε´ and ε´´ and loss tangent tan δ were calculated. The results prove validity of the proposed equivalent circuit containing parallel RC elements connected in series where the first RC element represents the bulk of material and the second RC element belongs to the double layer at the metal interface.1 Introduction Ferroelectric materials, such as crystalline lithium niobate, are very useful for fabricating a variety of optical and electronic devices, including optical switches and modulators, frequency switching devices, polarized controllers, pulsed waveguide lasers, surface-acoustic-wave filters and acousto-optic devices. Other useful properties of these materials include their piezoelectric, elasto-optic and pyroelectric characteristics. Lithium niobate-based devices are typically manufactured from bulk crystal material -usually a wafer about 0.5 to 1 mm thick -even though the devices use only a small fraction of the material's surface volume. The most important progress has been made in the 1960s when a high quality optical grade LN material started to be prepared [1]. The useful, well known, bulk single crystal properties can be also expanded to unique features of thin films. Furthermore, the thin film approach gives the possibility of using various substrates with alternate utilities [2]. As an alternative, scientists have attempted to produce thin films of crystalline lithium niobate. LN thin films are also known for their technological applications in electrochromic displays or in all solid state thin film microbatteries [3,4]. The methods used for preparation of LiNbO 3 thin films include MBE, laser ablation, sol-gel deposition, rf sputtering and, very recently, e-gun evaporation. Thin films formed by rf sputtering or e-gun evaporation show lithium deficiency [3,4]. The deposited films are cured by "Li doping" in order to solve this problem. The Li doping of deposited films leads to considerable increase of the conductivity and such values as 10 -4 Sm -1 can be reached. The behaviour of LN is affected by phase transitions. The samples of LN show such phase transitions. A transition usually appears at high temperatures (over 1000 °C) and one or more phases appear in doped samples at low temperatures (typically 60 to 130 °C) while the pure samples show no low temperature transition. It was pointed out that only very small amount of critical impurities may be enough for such phase transitions [1].