Abstract: The heat pipe reactor system based on the simple open Brayton cycle is with the characteristics of simple structure, inherent safety, and low risk of radioactive leakage, and is a potential leading technology option to meet the demand for more practical and innovative nuclear reactor designs capable of providing clean energy. The power-to-mass ratio of the heat pipe reactor is often used as an important indicator in the evaluation of engineering design. In this paper, we established a model for the power-to-mass ratio of the heat pipe reactor to explore the effect of various key parameters on the power-to-mass ratio. The analysis of a design like the Westinghouse eVinci? heat pipe (HP) reactor shows that the power-to-mass ratio varies versus temperature difference in the heat transfer path, including difference between the heat pipe and the core structure, and that between the heat pipe and the heat exchanger. The optimal power-to-mass ratio is positively correlated with the maximum operating temperature limit of the core structure material. In the future, more detailed modeling of compressors, turbines, heat pipes could be carried out to improve the accuracy of models.