The new method for improving photocatalytic activity of commercial titanium dioxide (anatase, Police, Poland) is presented. The modification was conducted under elevated pressure in ammonia water atmosphere in pressure reactor. It leads to built in nitrogen in titanium dioxide structure without surface hydroxyl group’s reduction. The photocatalyst was characterised by XRD, FTIR/DRS, UV-Vis/DRS. New N-doped titanium dioxide photocatalyst had also better adsorption of dyes than unmodified material. The analysis of OH radicals formation on the photocatalyst surface under UV and Vis irradiation were performed by fluorescence technique with using coumarin, which reacts with OH radicals to produce highly fluorescent product, 7-hydroxycoumarin. Under Vis light irradiation N-doped titanium dioxide photocatalyst produced about seven times higher amount of hydroxyl radicals than unmodified titanium dioxide. The photocatalytic activity of modified titanium dioxide was tested during three azo dyes decomposition (monoazo dye – Reactive Red, diazo dye – Reactive Black and poliazo dye – Direct Green) under UV and Visible light irradiation. The photocatalytic activity of N-doped titanium dioxide was always higher than unmodified one as well as P25 of Degussa.