TiO₂nanoparticles were prepared using hydrolysis of titanium tetraisopropoxide (TTIP) in W/O microemulsions consisting of water, nonionic surfactant, and cyclohexane. The physical properties of nanosized TiO₂have been investigated by TEM, XRD, FT-lR, TGA and DTA. In addition, the photocatalytic degradation of p-nitrophenol has been studied by using batch reactor in the presence of UV light in order to compare the photocatalytic activity of prepared nanosized titania. TiO₂ particles calcined at 500℃ have a stable anatase phase which has no organic surfactants. Above 300℃, the product completely transforms into the anatase phase and the rutile phase begins to appear at 600℃. The crystallite size and crystallinity increase with an increase of calcination temperature. The particles are shown to have a spherical shape and have an uniform size distribution. In adition, the size of particles increases with an increase of W?? ratio and a decrease of hydrocarbon chain length. In the photocatalytic degradation of p-nitrophenol, the titania calcined at 500℃ shows the highest activity on the photocatalytic degradation of ρ-nitrophenol and the pure anatase structure.