Abstract. Design optimization of a LHP system for a space application is considered. The system is composed of the LHP itself, an interface with the heat source (saddle) and a radiator. The criterion is minimal system mass while meeting the operational requirements. The optimization is performed with simultaneous consideration of hot and cold conditions with respect to imposed heat loads to the evaporator and external heat fluxes over the radiator panel. The design parameters of the system optimized are the active length of the evaporator, internal and external diameters of the primary wick, volume and size of the reservoir, thickness and width of the saddle, diameters and tube thickness of the transport lines and condenser, length of the condenser, dimensions of the radiator panel and the amount of the LHP working fluid charged. The LHP mass and optimal design parameters are obtained for three working fluids: armnonia, propylene and acetone; a comparative study of the optimal mass characteristics is performed. Fixed parameters are the required values of transferred heat loads, incident external heat fluxes for the hot and cold cases, length of the transport lines, material and fluid properties. Constraints include temperature limits for the attached equipment and the capillary limits of the LHP. A special steady state mathematical model was developed for the calculation of the LHP performance parameters and a global search metaheuristic, called Generalized Extremal Optimization (GEO), is used as the optimization tool.