A new NOx storage-reduction electrochemical catalyst has been prepared from a polycrystalline Pt film deposited on 8 mol% Y2O3- stabilized ZrO2 (YSZ) solid electrolyte. BaO has been added onto the Pt film by impregnation method. The NOx storage capacity of Pt-BaO/ YSZ system was investigated at 350℃ and 400℃ under lean conditions. Results have shown that the electrochemical catalyst was effective for NOx storage. When nitric oxides are fully stored, the catalyst potential is high and reaches its maximum. On the other hand, when a part of NO and also NO2 desorb to the gas phase, the catalyst potential remarkably drops and finally stabilizes when no more NOx storage occurs but only the reaction of NO oxidation into NO2. Furthermore, the investigation has clearly demonstrated that the catalyst potential variation versus temperature or chemical composition is an effective indicator for in situ following the NOx storage-reduction process, i.e. the storage as well as the regeneration phase. The catalyst potential variations during NOx storage process was explained in terms of oxygen coverage modifications on the Pt.