Trace Se(IV) can catalyze the discolouring oxidizing reaction of RDB with potassium bromate in nitric acid medium, and the conditions of kinetics has been studied in detail. A new method of kinetics spectrophotometry for determination of trace (IV) is established. The sensitivity of the method is 0.905 microg x L(-1). The range of measurement is 0-9.6 microg x L(-1). The method has been used for determining trace Se(IV) in Chinese herbal medicine with satisfactory results.