Whether CuO reduction is one step or two steps reaction has been argued for several decades, and one step reaction mechanism is mainly supported by CuO CV (cyclic voltammetry) profile, where only one cathodic peak represents the direct reduction from CuO to Cu. However, some researchers support two steps reaction mechanism, because they observed Cu2O as intermediate with spectroscopy technique or two cathodic peaks in the CV profile. In this study, two colloid electrodes containing crystal CuO particles are electrolyzed in KCl electrolyte at -1.7 V and -1.65 V respectively, and both of the products are composed of Cu2O and Cu. The electrolysis voltage can change the weight ratio of Cu2O and Cu, and the electrolysis results firmly prove that CuO reduction is two steps reaction: CuO is reduced to Cu2O, then to Cu. Therefore, the rationality of the criterion for determining one step reaction is re-considered in this study. The experimental conditions and parameters, including CV scan rate, atom arrangement of CuO, particle size of CuO, and anions and pH value of electrolyte, are discussed in the present study, and all these factors can affect CuO reduction process, thus influencing the cathotic peaks, such as the number, shape and potential of the peaks in CuO CV profile. It could lead to incorrect conclusion, when CuO reduction steps is determined by the cathode peak without comprehensive consideration of the effect from all these factors. In another word, cathodic peak number in the CV profile sometimes is not a straightforward and appropriate criterion to determine the number of steps of CuO reduction. In addition, S2- ions is found to be an effective addtive to enhance the cyclic ability and discharge plateau of Cu alkaline battery in this study. (C) 2018 The Electrochemical Society.