Abstract: In order to obtain the heat transfer performance of high temperature sodium heat pipe, the start-up performance and isothermal performance tests of sodium heat pipe under vacuum conditions were carried out, and the start-up speed and isothermal performance data of sodium heat pipe under vacuum conditions were obtained. Heat transfer performance tests under the forced cooling conditions were carried out, and the sonic limit characteristics of sodium heat pipe and the maximum heat transfer power under test conditions were obtained. The experimental results show that the developed high temperature sodium heat pipe is fully started at 580 ℃ under vacuum condition, and the start-up time is 20 minutes, the axial wall temperature difference is lower than 11 ℃, and the isothermal performance is good. The heat transfer power of sodium heat pipe is limited by the sonic limit in the temperature range of 500-650 ℃, and is limited by carrying limit in the operating temperature above 650 ℃. When the working temperature is 750 ℃ and 850 ℃, the measured maximum heat transfer power of the heat pipe is 4.78 kW and 8.02 kW, respectively, and the maximum axial heat flux density was 1.51 kW/cm² and 2.53 kW/cm², respectively. The test results show that the developed sodium heat pipe has strong heat transfer capability which can meet the needs of heat pipe nuclear reactors and other engineering applications.