Nanosize titania soi mas prepared from titanium tetraisopropoxide (TTIP) and conditions for the formation of stable sol were identified, As the H+/TTIP mole ratio decreased and H2O/TTIP mole ratio increased, stable sol was likely to be formed, The size and crystallinity remained unchanged after hydrothermal treatment of the stable sol at between 160 and 240 degrees C, However, hydrothermal treatment of unstable sol produced rod-like particles and crystallinity of particles was changed from anatase to rutile. This difference in phase transformation at low hydrothermal treatment temperature was likely to be caused by the fact that stable sor remained to be-stable even after hydrothermal treatment, while unstable sol had a tendency to be aggregated.