Recent progress of energy conversion system using magnetohydrodynamics (MHD) are reviewed. CO₂ enhanced oil recovery (EOR) power generation system with MHD generator is proposed. The plant efficiency of CO₂-EOR with MHD generator is higher than that of CO₂-EOR with gas turbine generator. It indicates that the plant efficiency of the system depends on the entrance gas temperature at the generator. To minimize in-flight time, nuclear electric propulsion (NEP) system with closed cycle MHD power generator is suggested for human exploration to Mars. It suggests that the size of the waste thermal energy with the radiator is important for improving specific mass of the NEP system. From these results, the minimum specific mass of NEP system is estimated to be less than 2kg/kW at the dependences of the radiator temperature and the enthalpy extraction ratio of MHD generator.