Abstract: A radioaaaiytical method has been developed for the determination of cerium-141 in mixed fission products. The method is based on the solvent extraction of cerium from 6N HNO3-0.5M NaBrO3 solution with substoichiometric amount of di (2-ethyl-hexyl) orthophosphoric acid (HDEHP) and subsquently measuring the radioactivity of the extracts.The influence of the concentration of HNO3 and HDEHP on the substoichiometric extraction of cerium has been examined. It is found that the optimum concentrations of HNO3 and HDEHP are 6M and 0.01M respectively. The effects of the interference of other ions on the determination have also been measured. Zirconyl and fluoride ions interfere seriously, but the presence of Ca2+, Sr2+, Fe3+, La3+, Nd3+, UO2 2+, Mo(Ⅵ) does not cause significant error in the results.This method does not require the determination of chemical yield, therefore it is rapid and easy to use. The relative standard deviation of the determination is ±1.3%.