This paper presents thermodynamic optimization of a bottom cycle which uses water and organic fluids as working fluids. The heat recovery steam generator is modeled with regard to different pressure levels, including the reheat of first pressure level, the configurations of heat-exchangers network where heat can be exchanged between flue gas and working fluids. Water is chosen as the working fluid of the first pressure level while an organic fluid for the second pressure level. Thermodynamic optimization of efficiency of the bottom cycle was conducted considering the variables of the heat-exchangers' inside the heat recovery steam generator HRSG and the operating parameters of working fluid of each pressure level and reheat. A genetic algorithm and a gradient optimization method were used with the thermodynamic model implanted in Matlab. It is shown that by using parallel and serial configurations of heat-exchangers and water in the first level pressure and organic working fluids in the second pressure level a better thermodynamic efficiency of the bottom cycle can be achieved. (C) 2018 Elsevier Ltd. All rights reserved.