The supercritical CO2 (SC-CO2) Brayton cycle has recently been gaining a lot of attention for fossil fuel power plants, nuclear reactors, and concentrated solar power applications. The current emphasis on such an innovative cycle is directed toward its economic evaluation and equipment sizing requiring process simulation and optimization. However, the rationale for choosing an appropriate thermodynamic model for supercritical CO2 does not receive much attention. Despite the important and fundamental role played by the thermodynamic model in the process simulation and optimization, its exactness as well as its impact on machinery sizing is not yet well studied. This paper intends to present a rigorous selection procedure of an equation of state for SC-CO2 as well as to investigate its influence on the performance and design of a SC-CO2 Brayton cycle.