Abstarct: A few limited researches have been done on heat recovery with application of high temperature heat pipes, especially on the replacement of heat pipe heat exchangers with Ljungstrom in steam power plants. The temperature of the smoke passing through the Ljungstrom in the steam power plant is around 320 ℃. By accurate and technical evaluation, the important parameters in the design of heat pipes can be determined and used them in energy recovery process in industry and also the replacement of Ljungstrom in steam power plants. For this purpose, 304 stainless steel heat pipe and suitable working fluid, which is a special oil for high temperature, have been used. In this study, a set of parameters such as flue gas temperature passing through the evaporator at five levels of 275, 300, 325, 350 and 375 °C, the aspect ratio at three levels 11, 18, 25 and the filling ratio of the heat pipe at four levels 30, 50, 70, 90% have been experimentally reviewed. The operating limitations of heat pipe are investigated and a thermal model for simulation of heat pipe is presented in this study. By increasing the filling ratio of the heat pipe, the thermal performance of the system in all aspect ratios is reduced. The best performance of the heat pipe in the experiments is obtained at the filling ratio of 30%, the aspect ratio of 18 and in the operating temperature of 325 °C. At this optimum conditions, heat transfer rate is 262.13 Watts and heat resistance of thermosyphon is 0.41 °C/Watt. The proposed thermal model simulates the thermal behavior of a system well with around 21% error.