Abstract: Based on optimization and analysis of a closed Brayton cycle system with He-N₂ as the working fluid, performance parameters of various components in the system were obtained under the condition when highest system efficiency is reached. On above basis, a preliminary design was carred out for the components including turbomachineries and heat exchangers. Taking the costs of turbomachineries and heat exchangers as the reference data for the cycle with pure helium working medium, relative costs of these components were calculated at different mole fractions of helium by using approximate relations between the equipment cost and their geometric parameters. According to the weight ratio of the costs of turbomachineries and heat exchangers, the relative total costs of the Brayton cycle with different molar ratios of helium were then obtained. Results show that when the mole fraction of helium gas lies in 50%-60%, there exists a minimum value of relative total cost, 12% lower than the pure helium case, which provides a better choice of working medium for closed Brayton cycle systems.