In this paper, photocatalytic decolourization of direct orange dye solution was made in the presence of commercial ZnO as catalyst under the artificial light (high pressure mercury lamp) as UV A source. Moreover the light intensity was measured. The decolourization of direct orange dye process was obeyed to pseudo-first order kinetics. Activation energies for dye was found to be 8.964 KJ mol–1 and the reaction is very fast, endothermic and spontaneously. The best conditions of decolourization for this dye was such as: initial dye concentration is 40 mg/L, best dose of ZnO is 350 mg/100 mL and the initial pH of an aqueous solution of dye is 9 were studied.