Abstarct: In the last century, energy has become particularly important due to the scarcity of energy resources and also the high amount of waste. Optimal use of energy and minimizing its waste rate has become one of the greatest goals of researchers and has provided the basis for the construction and production of equipment to achieve this goal. A heat pipe is a two-phase heat transfer device with a very high heat transfer rate that operates baxsed on the evaporation and density of circulating trapped fluid in a closed double-ended pipe. The main purpose of this study is to investigate a heat pipe without wick or so-called two-phase thermosyphon closed medium or high temperature that can be used in steam power plants. Due to the fact that there is no information about high temperature thermosyphons that can be used in the desired temperature range in power plants (100-350 ° C). After creating a computational network in Gambit software, the thermosyphon is simulated in Fluent software using the VOF model. In this study, first the thermosyphon studied by Fadhl is studied and its results are used to validate and check the accuracy of numerical simulations. The high temperature thermosyphon is then simulated with an oil fluid. The contours of the boiling and evaporation process of each of the filling ratios are given in the results section. The steady state contour for the 30% filling ratio is also shown. This contour is obtained 84 seconds after the start of the process. After reaching the steady state, the efficiency of this thermosyphon was calculated. The oil used in the closed two-light thermosyphon system works well for the intended application and increases the efficiency of the system by about 96%. In this research, how to build a high temperature heating pipe system with mercury operating fluid is described and the CFD simulation of the constructed heating pipe is discussed. Factors affecting the heat transfer of the heating pipe, including the internal heat transfer coefficients of the evaporator and condenser and the thermal conductivity, have been investigated experimentally and numerically, and the results have been reported graphically. The results show that the heat pipe has a flux of 0 and has the lowest heat resistance (271 watts of heat with an efficiency of 13 R = 1.3 ° C / W) against heat transfer, which has a better performance than lower input heat fluxes. Heat pipes made in the temperature range close to the design conditions offer more acceptable performance.