The sensing properties of the planar mixed potential CO sensor coupled with ITO as the sensing electrode has been investigated. It was found that the sensing properties of the fabricated sensor ITO/ScSZ/Pt could be greatly influenced by the thickness of the ITO sensing film. As the thickness of ITO sensing layer is reduced to micrometer range, the sensitivity of the sensor to CO was significantly enhanced. However, if the thickness is decreased further to submicrometer scale (i.e. ∼0.5μm), the sensor showed much lower sensitivity and sluggish response to CO. The possible reason could be related to the decrease of the effective area of the triple phase boundary and higher charge carrier transport resistance due to the much thinner porous ITO film.