Nano-SnO and normal SnO anodes for Lithium rechargeable batteries have been investigated by electrochemical impedance spectroscopy (EIS). Three types of equivalent circuits have been used to fit the spectra at different discharge states. The variations of impedance spectra, charge-transfer resistance and double layer capacitance have been discussed. The electrochemical reaction mechanism of SnO anode for Lithium ion batteries can be deduced as follows: a passivating film forms on the surface of SnO particles firstly, then Lif ions pass through the surface film and react with SnO to produce amorphous Li2O and to form fine grains of Li-Sn alloys in the core region underneath the passivating film.