Abstract: By using computational fluid dynamics (CFD) software and introducing Boussinesq approximate model to the volume of fluid (VOF) model, numerical simulations were conducted on the gravity heat pipe under different heating power with water as the working medium, so as to study the flow and heat-transfer characteristics of the gravity heat pipe based on experimental, theoretical and simulation data. Results show that the wall temperature obtained by CFD simulation agrees well with that of experiment, and the CFD visualization can be used to qualitatively analyze the effects of heating power on the heat-transfer characteristics of gravity heat pipes. In the range of 10-80 W, with the increasing of heating power, the heat-transfer coefficient at evaporation section firstly increases and then decreases, while that at condensation section keeps increasing. Once the heating power exceeds a certain value, the heat-transfer performance at evaporation section deteriorates, when a dried up liquid pool could be observed through CFD visualization, indicating a heat transfer limit is already reached.