Nano-sized materials, have much bigger surface-bulk ratio than coarse materials. Thus, surface properties become paramount, which makes these materials particularly appealing in applications in which such properties are exploited, as in gas sensors. The purpose of this work is that investigates the gas sensing characteristics of the nanocrystalline ITO thick films with various particle sizes. At first, we synthesized nano-sized ITO powders with particle size of 15, 30 and 70nm in an equivalent diameter via coprecipitation method. And then, the sensitivity of ITO thick films was measuresd for ethanol(C2H50H) as target gas and some Volatile Organic Compounds(VOCs) such as Toluene, Methanol, Benzene, Chloroform. XRD, BET, TEM, SEM, XPS were used to investigated characteristics of ITO powders and thick films.