Abstract: In this paper, the simultaneous determination of U and Np by polarography is described. In the supporting electrolyte of 2mol/l HAc-2mol/l NH4Ac, the polarographic waves of U6+ and Np5+ show characteristic reversible waves. The half wave potential vs Hg pool electrode is -0.64 V for U6+ and-0.95 V for Np5+,with a difference of 0.31 V; so they could be determined simultaneously. Besides, the electrode reactions correspond with Nernest Equation and Ilkovich Equation, and the electron number and coefficient of diffusion could be calculated. This method can be used for the analyses of U and Np in Fluoride Volatilization Process for reprocessing spent fuel.