The sensing characteristics of new oxide sensing materials, NiO, NiO-YSZ and CuO, were evaluated at the temperature of $700^{circ}C$ in 10 % $O_2$containing atmosphere. Through simultaneous response to $NO_2$ and NO, the sensing mechanism of oxide electrode was studied and the relation of EMF and NO/$NO_2$ concentrations was elucidated. Moreover, for highly sensitive NOx sensor, modified mixed potential sensor which has at least two oxide electrodes was proposed.