High temperature co-electrolysis (HTCE) technology provides a very promising way to store energy and utilization of carbon dioxide. In this paper, the overall efficiency of the HTCE system was calculated through electrochemical and thermodynamic analysis. A thermodynamic model in regards to the efficiency of the HTCE system was established and the effects of five key parameters, electricity generation efficiency of high temperature gas-cooled reactor (HTGR) (eta(el)), electrolysis efficiency of CO2, electrolysis efficiency of H2O, thermal efficiency and co-electrolysis temperature on the overall efficiency were investigated in detail. The results showed that although the introduction of CO2 increases the complexity, it does not significantly reduce the overall system efficiency. Analysis of the impact coefficients gives the order of the impact factors. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.