Abstract: Photocatalytic degradation kinetics of 4BS dye stuff was studied in an open batch slurry reactor with fine particulate TiO ₂ catalyst and UV light. The influences of mixing of solid-liquid and external diffusion on degradation rate were eliminated by increasing stirred rate. Favorable pH was about 3 5. Distance of light source with surface of solution was also important for photocatalytic reaction, but the effect of intensity of illumination was very complicated, so a special and acceptable light intensity was selected by changing the position of lamp. Then the experiments by changing the amount of catalyst, reaction time and initial concentrations of reactant were made.A suitable amount of catalyst was about 1 0 g·L ^-1 , and the higher the initial concentrations of dyestuff, the lower the degradation rate. According to numeric analysis for the above experimental results, the reaction kinetics was second order with respect to the concentration of 4BS, and the reaction rate was proportional to the amount of catalyst under the experimental condition. However, the rate constant was different for the catalyst amount in 0 5 g·L ^-1 or in the range of 1 0-6 0 g·L ^-1 , which was due to the change of determinanf factor for degradations. The reaction kinetics that could be used for all of the degradation process was necessary in the reactor design.