Liquid cooling with phase change has been demonstrated to be a very efficient technique for thermal management of electronics because it has the potential to achieve high heat transfer coefficients compared to single phase liquid cooling. Previous studies on liquid immersion cooling with fluorocarbons have shown the effectiveness of boiling enhancement structures in lowering boiling incipience, raising the critical heat flux, and reducing evaporator size. Two-phase thermosyphons are an alternative to liquid immersion cooling, where phase change liquid cooling can be implemented within a closed-loop device. The present study involves a two-phase thermosyphon with boiling enhancement structure in the evaporator, which is subjected to subatmospheric pressures for lowering the saturation temperature of the working fluid. The objective of the present research is to provide a detailed understanding of the effect of liquid-fill level on boiling of water with enhancement structures at subatmospheric pressures. The study will take into account the influence of system pressure and enhancement structure geometry on the boiling heat transfer. Experiments were performed at three different pressures, , , and , using a stacked enhancement structure with three different geometries (one, four, and six layers), corresponding to three different liquid-fill levels (fill ratios of 0.5, 0.7, and 0.9). The results are compared with a base line study on subatmospheric pressure boiling from a plain surface at similar liquid-fill levels.