Tin-doped indium oxide (ITO) is an important industrial material, which was extensively studied as an electrode material for optoelectronic devices and semiconductor gas sensors. However, its application as an oxide electrode material in mixed-potential gas sensor has not been explored so far. In this report, the sensing properties of the planar scandia-stabilized zirconia based mixed potential gas sensor coupled with hydrothermally-obtained nano-sized ITO as an electrode material have been investigated. The planar sensor has shown good sensitivity to low-level of CO concentrations (16~500ppm) in a lower oxygen ambient (4 vol.%) in terms of the high magnitude of the signal and fast response (<5s). A possible sensing mechanism of the planar sensor for rapidly detecting CO has also been discussed. The fast response feature of the fabricated sensor could be attributed to the porous nanocrystalline ITO employed as a thick film electrode in the CO sensor.