Abstract: The loop heat pipe (LHP), featuring high reliability, low thermal resistance, great thermal transfer capability, high efficiency, and good thermal equivalence, is widely used for the thermal control of satellites, spacecrafts, and electronics. It can dissipate a large amount of heat while keeping a tight control of the heat source temperature, especially for the miniature LHPs as used in flat evaporators, therefore, it has a good prospect of potential applications. This paper builds a numerical thermodynamic model and a dynamic model for simulating the operation of LHPs. The calculation program is developed with C++ language. The calculation of the thermal operating characteristics of a given LHP with a flat evaporator is carried out. It is shown that the pressure drop within the flat evaporator is dominant in the total pressure drop of the loop.