Double reheat steam systems have been receiving more attention because of the rapid development of ultra-supercritical power plants. In this study, the thermodynamic analysis and design optimization of a double reheat system in an ultra-supercritical power plant are comprehensively conducted. Besides, thermodynamics calculation, as well as exergy and techno-economic analyses are conducted to reveal the energy-saving effects of various systems. Through comprehensive system optimization, an optimized double reheat system adopting 10-stage extractions and two outer steam coolers is proposed, whose heat rate can be further reduced by 80.7 kJ/kWh (1.04%) based on conventional double reheat system. Considering that the additional investment of the optimized double reheat system is only increased by 0.76%, the cost of electricity of the optimized system is only 55.89 USD/MWh. This cost is not only less than the conventional double reheat system, but also even less than the single reheat system. This result indicates that system optimization can improve both thermal and economic performance. The research of this paper may provide an effective method for the optimization of a double reheat system in ultra supercritical power plants. (C) 2014 Elsevier Ltd. All rights reserved.