The liquid lead-bismuth eutectic (LBE) is an ideal primary coolant for the fourth-generation advanced nuclear systems. Due to its special physical properties, especially the low Prandtl number and high density, the natural convection phenomenon caused by buoyancy is significant. In this study, a circular tube is three-dimensionally modeled to predict the flow and heat transfer process of LBE. On the base of the model validation, simulations of the uniformly heated tube with/without buoyancy are performed. The different thermal-hydraulic performance obtained under different tube arrangements and operating parameters are compared, and the effects of buoyancy on the convective heat transfer of LBE are discussed. The results show that for LBE flow with smaller Reynolds numbers, convective heat transfer is significantly enhanced when gravity is opposite to the flow direction, while the same gravity and flow direction slightly weakens the convective heat transfer capacity. For the higher Reynolds number LBE flow, the buoyancy impact and the tube arrangement can be ignored. When the tube is placed horizontally, the heat transfer coefficient of the lower wall is about three times that of the upper wall. For different engineering applications, an appropriate arrangement is necessary to improve the efficiency of heat exchanger. This study may contribute to the development and application of LBE-cooled reactors.