In the development of liquid metal Rankine cycle system, experiments using liquid metals generally cannot be conducted on the International Space Station (ISS) or the space shuttles due to a number of issues including safety. Some other fluids must be used to simulate potassium flows in a Rankine cycle. In this paper an analysis for the simulation using surrogate fluids is made. The results of the analysis indicate that single-phase liquid metal flows, either liquid or vapor, can be simulated using surrogate fluids both in ground and space experiments. For two-phase liquid-metal flow, a credible simulation with a surrogate fluid is currently not possible. For the problem of vapor-production efficiency in a boiler, a simulation using a surrogate fluid whose properties are similar to potassium is workable. A high concentration of copper nanoparticles in water (Cu-water nanofluid) is considered a potential possibility for a fluid to simulate liquid-metal turbulent flow and the boiling process. In the final stage of developing the liquid-metal Rankine-cycle system, experimental tests in a subscale liquid-metal system are still indispensable.