Nanocrystalline indium tin oxide (ITO) powders with different tin contents ranging from 0 to 8 at% were prepared by a coprecipitation process. A mixture of rhombohedral and cubic structured nanocrystalline ITO powders was obtained, and the amount of the rhombohedral ITO increased as the content of Sn increased. During heat-treatment the rhombohedral ITO was transformed to cubic around 900°C. The phase transformation behavior of the powders was examined as a function of oxygen partial pressure. When the rhombohedral ITO powder was heat-treated in a low oxygen partial pressure (nitrogen gas) atmosphere the phase transformation was accelerated and this acceleration can be attributed to the formation of oxygen vacancies that accelerate atomic transport in the rhombohedral ITO.