Abstract: Power conversion unit is an important part of high and very high temperature gas-cooled reactors (HTGR and VHTR). The power conversion unit coupled with either HTGR or VHTR was studied in this paper. Based on four key parameters (reactor outlet temperature, reactor inlet temperature, compressor pressure ratio and main steam parameters), five cycle strategies were compared. Considering several engineering limits, the combined cycle with topping closed Brayton cycle and bottoming reheat Rankine cycle is more competitive. Its bottoming cycle should be subcritical in range of HTGR, and in range of VHTR the bottoming cycle should be supercritical. Combined cycle can efficiently convert heat of HTGR and VHTR, and its reactor inlet temperature is within the allowable range of the material of reactor pressure vessel, which has sufficient safety.