The electrochemical impedance spectroscopy was employed primarily to explore its potential as an in situ monitor of film thickness and microstructure during the oxide formation and growth on a pure zirconium in water. The oxide film growth was accelerated by anodization of zirconium metal in a buffered alkaline solution at room temperature. AC impedance was measured at corrosion potential at various cumulative anodic charges. The impedance data suggests that the oxide film can be modeled adequately by a parallel RC circuit with the concept of a constant phase element. The oxide thickness estimated from the AC impedance was observed to increase linearly with the cumulative anodic charge indicating that the current efficiency for oxide growth is only about 6 percent under the condition. The oxide thickness analyzed by Auger Electron Spectroscopy (AES) corresponded with that predicted from the anodic charge. Hence it is suggested that the AC impedance techniques can be used to monitor the inner imprevious layer of the Zr oxide.