Formation of solid electrolyte interface (SEI) on the surface of graphite in Li/graphite cells was studied using the electrochemical impedance spectroscopy (EIS) technique. Results show that EIS of the cells varies considerably with cell voltage. In particular, resistance (R sei) of the SEI increases significantly with lithiation and decreases reversibly in the subsequent delithiation in a narrow voltage range between 0.15 and 0.04V. According to the change of the R sei during the first lithiation process, we might divide the formation of the SEI into two voltage regions: the first region generally occurs above 0.15V and the second region below 0.15V. The SEI formed in the first region is less conductive, and that formed in the second region is highly conductive. It is shown that ionic conductivity of the preliminarily formed SEI is greatly affected by the kind of solvents and salts of the electrolyte. We observed that the R sei increases in the order of LiBF4 >LiSO3CF3 >LiBOB>LiPF6 for salt, and of NMP>EMC>MB for solvent. Addition of vinylene carbonate into the electrolyte results in a significant increase of the R sei.