The variation of impedance property during the 1st charge/discharge course was investigated by electrochemical impedance spectroscopy (EIS). The results show that the proposed equivalent circuit of RS(Qdl(RCtZW))(QSEIRSEI) perfectly simulating the EIS measurement data and all the errors less than 5% can reasonably explain the course of the reaction between Li and NiO electrode. The SEI film which distinctly influences the electrochemical performance of lithium ion battery is free at open circuit voltage. It begins to slowly grow at polarization voltage of 1.5V and completely forms at 0.3V in the 1st discharge. To react further with NiO, the Li+ must pass through SEI film and generate the fine grains Ni and amorphous Li2O matrix. SEI film partially decomposes and the NiO and Li are formed by reversible reaction of Ni and Li2O in the charge course.