Copper-doped tin oxide nanopowder has been synthesized via the hydrothermal route, in which pure metallic Sn, diluted nitric acid solution and Cu(NO3)2 are used as the starting materials. The hydrothermal treatment at about 200°C for 10 h results in rutile crystalline SnO2 particles with a narrow size distribution typically in the range of 3~6 nm. The average crystallite size of 5 wt% CuO-doped SnO2 particles remains smaller than 12.5 nm even after annealing at 800°C. The evaluation of the sensitive properties of the synthesized powder with various amount of CuO doping is conducted on the thick-film samples fabricated by screen-printing method. The high sensitivity toward H2S as shown by the sensor test results show the possibility of using this material for gas sensor fabrication.