Supercritical CO2 Brayton cycle has emerged as an alternative power block for Concentrated Solar Thermal (CST) systems and nuclear plants. In order to optimise the thermal performance of the Supercritical CO2 Brayton cycles, a combination of high pressure and temperature are required. The high pressure of the S-CO2 Brayton cycles has a remarkable effect on not only the thermal performance of the cycle but also the thermodynamic and mechanical performance of the solar receiver. In this paper, three S-CO2 Brayton cycle configurations without reheat are proposed by introducing an ejector prior the heater, which reduces the pressure at the solar receiver. A comprehensive thermodynamic analysis and a multi-objective optimisation were performed to study the thermodynamic feasibility of the proposed cycles. The effect of the cycle pressure ratios, turbine split ratio, turbine inlet temperature and exit pressure of the ejector on the thermal performance and output parameters of the S-CO2 Brayton cycles assisted by an ejector was analysed. The proposed configurations were compared with the conventional S-CO2 Brayton cycles without reheat and referenced steam Rankine cycles (projected thermal efficiencies of 0.416-0.47 in 2020-2025). The results showed that under some operating conditions, the proposed configurations assisted by an ejector can achieve higher efficiencies than the referenced steam Rankine cycles. As the ejector exit pressure increased, the thermal performance of the proposed configurations approached the conventional supercritical CO2 Brayton cycles. (C) 2016 Elsevier Ltd. All rights reserved.