This paper presents optimization of a supercritical carbon dioxide Brayton cycle for three types of 300-MWth small modular reactors (SMRs); a pressurized water reactor (PWR), a sodium-cooled fast reactor (SFR) and a high-temperature gas-cooled reactor (HTGR). The parameters of the pressure ratio and the flow split fraction were examined for sensitivity analysis and optimization of cycle. The optimized cycle efficiencies of PWR, SFR, and HTGR were 30.6%, 46.38%, and 50.04%, respectively. Key components, i.e. turbomachinery and heat exchangers for the SMRs were designed to develop the optimized cycles. The cycle thermal efficiency was improved by using investigating the effects of the channel shape (zigzag, s-shape, airfoil fin) of the printed circuit heat exchangers (PCHEs) on the pressure drop. The study indicated that using airfoil fin type PCHE may increase the cycle thermal efficiency by about 1.0% in comparison with zigzag type PCHE. The effect of turbomachinery efficiencies on the cycle thermal efficiency were investigated. (C) 2018 Elsevier Ltd. All rights reserved.