An MHD gas turbine energy conversion system for mirror fusion reactors is proposed. An inert-gas working fluid, which has been heated in the blanket, is superheated in a plasma thermalizer up to 2800 ∼ 3000 K, where the charged-particle energy from the open ends of a mirror and from neutral-beam injectors is thermalized. – Overall-efficiency calculations show that the MHD gas turbine system shows great potentialities for overall-efficiency increase as well as for a reduction in the capital cost of a mirror reactor conversion system, and that the overall efficiency of the MHD gas turbine system becomes larger than that of the electrostatic Venetian blind direct energy convertor system with a steam-turbine bottoming cycle when the reactor blanket outlet temperature is higher than 1300 K.