Degradation of the organic coatings on steel has been investigated by the dynamic impedance measurements. The electrochemical impedance spectroscopy (EIS) has been used to evaluate the corrosion of steel coated by the organic film. Generally the electrochemical impedance of the coated steel shows complicated behavior, because the degradation of organic coating involves various steps until the steel corrodes under the coating. In the present paper, time variation of electrochemical impedance was measured to investigate the initial stage of coating degradation. The dynamic impedance was displayed on 3-dimension plots, whose axes were real and imaginary components and time. The time variation of the film resistance related to the permeation of water was monitored to discuss the corrosion mechanisms of coated steel. copyright The Electrochemical Society.