The result shows that the increasing the heat load higher than the optimum condition can reduce the thermosyphon performance (Figure 5). It can be argued that in a very high circulate regime caused by high energy input, the resistance of the produced film could have a negative effect on transforming heat from the evaporator to the condenser fluid. By applying more heat to the evaporator a thick liquid layer established in the condensation section. Increasing the thickness of liquid layer can cause higher thermal resistance and consequent lower performance of thermosyphon