The paper deals with selecting the mass flow rate of cooling water under variable load of a power unit such that optimum operating parameters of the unit can be achieved. The change in the cooling water mass flow rate influences the resistance to flow (on the cooling water side) and the pressure of the condensing steam. A higher cooling water mass flow rate increases the resistance to flow and power supplied to the cooling water pump, but it also results in a drop in the condensing steam pressure and an increase in the power generated in the LP (low-pressure) part of the turbine. Since the change in the mass flow rate affects the performance of the condenser, the LP part of the turbine, and the cooling water pump, a system comprising these components was analyzed. The cooling water mass flow rate was chosen via minimization of the total entropy generation rate in these components and a change in the unit's power output defined as a difference between the power outputs of the LP part of the turbine and the cooling water pump. The aim of this paper is to find the optimum value of the mass flow rate of cooling water under variable load of the power plant. (C) 2016 Elsevier Ltd. All rights reserved.