Indium tin oxide (ITO) nanopowders were prepared by a modified chemical co-precipitation process. The influence of different SnO2 contents on the decomposition behavior of ITO precursors, and on the phase and morphology of ITO precursors and ITO nanopowders were studied by X-ray diffractometry, transmission electron microscopy and differential thermal and thermogravimetry analysis methods. The TG-DSC curves show that the decomposition process of precursor precipitation is completed when the temperature is close to 600 ℃ and the end temperature of decomposition is somewhat lower when the doping amount of SnO2 is increased. The XRD patterns indicate that the solubility limit of Sn4+ relates directly to the calcining temperature. When being calcined at 700 ℃, a single phase ITO powder with 15% SnO2 (mass fraction) can be obtained. But, when the calcining temperature is higher than 800 ℃, the phase of SnO2 will appear in ITO nanopowders which contain more than 10% SnO2. The particle size of the ITO nanopowders is 15-25 nm. The ITO nanoparticles without Sn have a spherical shape, but their morphology moves towards an irregular shape when being doped with Sn4+.