Abstract: Closed-Brayton-cycle system is a feasible choice for future space power reactor as the power conversion system because of its high efficiency and high power density. In order to ensure the economic benefit, most of the land-use Brayton-cycle systems for high temperature gas-cooled reactor take the efficiency optimization. However, there is a volume/mass limit for the space use. So another optimization method, specific power optimization was presented. In this paper, a detailed comparison was made by Fortran language to research which optimization method could be the more possible choice for future space use. This paper will be helpful to understand and design the small space power reactor. The results show that specific power optimization has advantages in decreasing system’s weight and volume, but would increase design difficulty. The final design description may probably be proposed between the two methods according to the project’s demand.