The corundum structures of In
2
O
3
:Sn (ITO) nanoparticles were synthesized by hydrothermal processing of InCl
3
and SnCl
4
·5H
2
O precursor at low temperature of 250 °C and 40 bar pressure for 3 h. The precursor was precipitated in a white gel of InOOH. After drying at 150 °C in air, it was crystallized in orthorhombic structure. InOOH powder was transformed into dark-gray rhombohedral In
2
O
3
by sintering at 420 °C in forming gas for 1 h. The samples were characterized by means of XRD, SEM, and TEM. The particle size of the resulted ITO powder was about 32 nm.