Nanocrystalline TiO2 powders, doped in range of 0.05-5 mol% Nb and Ta, have been prepared using a chemical method. Synthesized powders have an anatase crystal structure and a fine particle size of 10-20 nm. Energy-dispersive X-ray microanalysis shows uniform dopant distribution in the powder. Temperature dependence of resistivity for both the Nb- and Ta-doped TiO2 sintered at 1300 degrees C exhibits n-type conductivity in the range 100-500 degrees C with an activation energy of about 0.2 eV. It was found by transmission electron microscope analysis that the dopants in the doped TiO2 sintered samples are segregated on grain boundaries after repeated high temperature treatment regardless of doping ranges. Addition of Zr in small amount (about 1 wt%) sufficiently suppresses such phase segregation, improving the high temperature stability.