We have prepared TiO2 nanoparticles by the hydrolysis of titanium tetraisopropoxide (TTIP) using HNO3 as a peptizing agent in the hydrothermal method. The physical properties of nanosized TiO2 have been investigated by TEM, XRD and FT-IR. The photocatalytic degradation of orange II has been studied by using a batch reactor in the presence of UV light. When the molar ratio of HNO3/TTIP was 1.0, the rutile phase appeared on the titania and the photocatalytic activity decreased with an increase of HNO3 concentration. The crystallite size of the anatase phase increased from 6.6 to 24.2 nm as the calcination temperature increased from 300 ℃ to 600 ℃. The highest activity on the photocatalytic decomposition of orange II was obtained with titania particles dried at 105 ℃ without a calcination and the photocatalytic activity decreased with increasing the calcination temperature. In addition, the titania particles prepared at 180 ℃ showed the highest activity on the photocatalytic decomposition of orange II.