The electrochemical impedance spectroscopy (EIS) was used to analyze the corrosion evolution of pre– and post–transition of the annealed Zr–4 alloy in 0.01 mol/L LiOH aqueous solution at 360 ℃/18.6 MPa. The results show that oxidation weight gain of Zr–4 alloy increases slowly before corrosion transition. When Zr is oxidized into ZrO2, the stress appears in the oxide film due to volume expansion. With gradually thickening of the oxide film, accumulation of micro–defects will relax the stress in outer layer of oxide film. As a result, the uniform and compact oxide layer turns into double lyer with a porous outer lyer and a dense inner one. Meanwhile the impedance spectra evolve from single arc into doble capacitive arcs. When the oxide film forms macro–defects such as cracks, corrosion trasition tkes place, giving rise to an accelertion of oxidation weight gain and the rapid drop of impedace nd corrosion potential. Corrosion potential and electrochemical impedace spectroscopy are able to obtain in situ some informatioof the trasition point.