Heat pipes and thermosyphons are reliable, fully passive, highthermal conductors and self actuated devices utilized for transportinglarge quantities of heat energy. Closed, gravity-assisted, two-phasethermosyphons are wickless heat pipes fabricated as sealed tubes thatcontain small amount of working fluid, applying the phase changecharacteristics by means of evaporation and subsequent condensation.As a high effective thermal conductors and isothermal devices thermosyphons have been widely used in energy and different industrial applications due to their simple construction, small thermal resistance ,broad operating limits and low fabrication costs.Thermal analysis of thermosyphon performance is theoretically and experimentally presented in the steady state operation of the closed two-phase water/copper thermosyphon. The main objectives of the current study are to develop a simplified, rapid and understandable engineering theoretical model for the thermosyphon operation. Anumerical, one-dimensional, steady tate flow model is substantially presented for the vertical, gravity assisted, closed two-phase thermosyphon. The model accounts for the cooling flow regimes, the heat transfer processes of individual sections, the thermosyphonthermal losses, the local change of the working fluid thermophysical roperties, the local heat transfer coefficient and correction for the expanded liquid pool height in evaporator. In addition, to validate thepredicted mathematical model results, an experimental investigation isultimately conducted. Also, The study investigates the effects of changethe operating parameters: heat load, cooling rate and liquid fill ratio onthe thermosyphon performance are considered. Furthermore, the axialvapor and outer wall temperature distribution, as well as the heat