Photocatalytic degradations of p-nitrochlorbenzene(p-NCB)with distilled water were investigated with ZnO crystals(catalyst)of 70nm in diameter under UV irradiation.The suitable experimental conditions are determined as:ZnO 0.25g,pH 7,p-NCB concentration 30mg/L.These variables in terms of the degradation rate have been discussed,which was defined as the rate of the initial degradation to the final degradation of p-NCB.When all of the experimental degradation rate values are plotted as a function of irradiation time,all of the points appeared on a single line for wide range of p-NCB degradations.On the basis of these results,it has been concluded that at lower ZnO catalyst amount,much of the light is transmitted through the slurry in the container beaker,while at higher catalyst amount,all the incident photons are observed by the slurry.Degradation rates of p-NCB were found to decrease with increasing solution pH.It has been concluded that the maximum degradation rate values of pNCB under principally the same experimental conditions mentioned above are 97.4 %,98.8% and 95.5% at 100min respectively.The results suggest that the photocatalytic degradation is initiated by an oxidation of the p-NCB through ZnO surface-adsorbed hydroxyl radicals.Absorption spectra are recorded using spectrophotometer before and after UV-irradiation in the wavelength range 200-400nm at room temperature.It is found that the variation of irradiation time over the range 20-100min resulted in change in the form of the spectrum linear absorption and a higher maximum value will be obtained at longer irradiation time.