A mathematical modeling of a system consisting of a cascade of a thermionic energy conversion (TIEC) device and an alkali metal thermal to electric converter (AMTEC) device has been performed to optimize the efficiency of the cell. The TIEC is heated by electronic bombardment, which converts heat partially into electricity and rejects the remaining heat. The AMTEC utilizes the rejected heat of the TIEC. Cascading these two cells provides lots of advantages. A mathematical model for the cascade converter has been developed to analyze the effects of key parameters such as power level, heat fluxes and temperatures. In this effort, a 12-node system of non-linear simultaneous equations has been constructed which is solved by MATCAD and a locally optimized efficiency has been derived. Thus, efficiency of the cascaded cell is improved, so that it is greater than the highest efficiency among the TIEC and AMTEC and lower than the sum of their individual efficiencies.