Nano-sized NiO powder was modified by the addition of BaO, La 2 O 3 , and CuO (10 wt% each) to change its morphology and electrochemical catalytic activities. Then, the NO x -sensing characteristics of mixed-potential-type YSZ-based planar sensor attached with each of the obtained NiO-based samples as a sensing electrode (SE) were examined at elevated temperatures. As a result, the CuO-added NiO SE was found to give a large enhancement in the NO 2 sensitivity as well as the NO 2 selectivity at 8001C even under a wet condition, in comparison with the pure NiO SE.