We proposed room-temperature-operated sensitive NO2 sensors by using a sol-gel thin-film device with a vertical channel structure. With the vertical channel, the low conductivity metal oxide clusters delivered large enough current. We hence noticed the unique sensing modulation characteristics, which were not observed in conventional lateral channel metal oxide devices, by simply adjusting the film composition. With a systematic study by tuning the composition and investigating the surface property, we proposed the level of hydroxyl group as the dominant factor to influence the sensitivity. Finally, the good-enough selectivity, repeatability, and shelf lifetime were successfully demonstrated.