Abstract: The start-up of a high temperature heat pipe is a complex multi-phase transition and flow heat transfer process, which may result in heat transfer oscillations, causing fluctuations in the heat pipe temperature and thus affecting the operational safety of the reactor. In this study, for the high temperature sodium heat pipe with a length of 1 000 mm and a diameter of 20 mm, the evolution law of the heat transfer oscillation was summarized through start-up experiments under vertical and horizontal conditions. The finite element thermal-mechanical simulation and thermal fatigue program analysis of a localized heat pipe reactor core design with generalized characteristics were carried out, and the stress response and the consequences of fatigue damage induced by heat pipe heat transfer oscillations were investigated. The experimental results show that the change of sodium evaporation rate causes the change of heat pipe pressure under vertical condition, which affects the evaporation and reflux balance of working medium, and causes the oscillation of heat transfer in different characteristics during start-up process. Enhancing the input power will shorten the frequency and decrease the amplitude of the oscillation. When the heat flow density increases from 9 973.71 W/m² to 12 452.28 W/m², the temperature oscillation amplitude decreases by nearly 30% and the oscillation period is shortened by about 40%. In the heat transfer oscillation state, the heat pipe will still maintain the heat transfer oscillation when the input power is suddenly reduced, but the waveform changes from a sawtooth shape to a trapezoid shape. The oscillation is obviously suppressed under horizontal condition. Simulation results show that, when the high temperature heat pipe heat transfer oscillation occurs, the thermal fatigue failure risk area of the localized reactor core is the inner wall of the evaporation section of the heat pipe. Fatigue damage is the largest in same-phase oscillation. The average value of the stress increases when the peak temperature difference increases due to the phase difference of oscillation, but because the decrease in the amplitude of stress of different-phase condition, the fatigue damage is reduced compared to the same-phase condition. Considering the effect of the conservatism deviation and creep on the fatigue damage, the thermal fatigue analysis is based on the safety factor K. Fatigue safety analysis results show that when the safety factor K is lower than 1.4 there is almost no risk of fatigue failure. K reaches 1.4 or more fatigue damage is gradually significant. When K=1.6 the fatigue life of the pipe wall material reduces to 3.29 years. This study is of great significance in analyzing the mechanism of heat transfer oscillation of high temperature sodium heat pipe and improving the reliability assessment of heat pipe.