In this study, analysis of an electric field sensor using an electro optical effect by distortion of a helical structure of a ferroelectric liquid crystal was carried out. When a sine wave electric field of commercial frequency (60 Hz) was applied to a ferroelectric liquid crystal and a transmitted light intensity in a crossed nicols optical system was measured, the amplitude of the transmitted light was proportional to the amplitude of the applied electric field. In order to pursue a molecular arrangement in the helical structure, a free energy of liquid crystal was evaluated and the electro optical effect was simulated by the pursued molecular arrangement. As a result, the linear property of the electro optical effect was explained with the molecular distortion.