We tested a thermosyphon loop with water as the working fluid using heating rates between 100 W and 400 W. Four kinds of core blocks were installed in the evaporator and tested: a hollow block, and blocks with narrow holes: Φ 2.2 mm × 90; Φ 2.5 mm × 55; and Φ 4.0 mm × 30. The temperature distribution indicated stable flow circulation inside the thermosyphon at low volume ratios but was unstable when the volume ratio was increased higher than 30%. The characteristics of the flow pattern are summarized as a flow map showing the heating rate versus the volume ratio. The recovered heat and the thermal resistance of the thermosyphon loop were clearly improved by using the core blocks with narrow holes instead of hollow blocks for the treated volume ratios from 20% to 80%. The thermal resistance increased when the volume ratio reached high values, suggesting that the effects from the abnormality of the flow circulation affected thermal resistance. The velocity of the gas stream in the thermosyphon was estimated by assuming an isothermal state, and it is diagrammed showing the heating rate at different temperatures. The current experiment of the thermosyphon loop is plotted in this diagram, which indicates the need for a wide margin due to the limitations of the sonic velocity and the pressure head at the full height of the heat pipe.