TiO2 nanoparticles were prepared through the hydrolysis of titanium tetraisopropoxide (TTIP) using TMA (tetramethylamine) as a peptizer via a hydrothermal method. The photocatalytic degradation of orange II was studied using a batch reactor in the presence of UV light. The particle sizes were similar after hydrothermal treatment at 120 ℃ ~ 170 ℃, but it increased to 23 nm at 200 ℃. The titania particles prepared at 170℃ had the highest activity, and the titania particles calcined at 600℃ also had the highest activity for the photocatalytic decomposition of orange II. The degradation rate of orange II increased upon increasing the catalyst weight, but no further increase occurred above 2.0g/L. In addition, the decomposition rate was higher under acidic conditions than under neutral or the basic conditions.