Cu oxidation state and Cu-inhibitor complex removal have been investigated by electrochemical measurements for developing the post Cu-CMP cleaner. To elucidate Cu oxide state, Sequential Electrochemical Reduction Analysis (SERA) is performed. As results, SERA could detect Cu2O and CuO independently and quantitatively. Furthermore, we measured Cu-inhibitor complex removal using Open Circuit Potential (OCP). OCP measurement could reveal difference of removal speed of Cu-inhibitor complex. So, SERA and OCP are effective to develop post Cu-CMP cleaner.