Liquid lead–bismuth eutectic (LBE) fast cooled reactor incorporates the primary coolant with a large margin to boiling and inert interaction with air or water. Its properties are very different from those of the reactor cooling fluids such as water and sodium in a number of ways, including density and thermal expansion. LBE has a greater potential for natural circulation, which would result in a better-performing design for passive reactor security systems. In this paper, the transition characteristics from the forced circulation to natural circulation flow driven by buoyancy are studied. The effects of heating power and initial flow rate on the natural circulation formation process are investigated. It is found that the greater the heating power, the shorter the time required for the establishment of the natural circulation. And the greater the initial forced circulation flow rate, the longer the time required for this process. The natural convective heat transfer in the test section is affected by the magnitude of the heating flux density. When under inclination conditions, the transverse component of buoyancy enhances the heat transfer of the LBE fluid by a maximum of more than 10%. The effects of inclination conditions on the heat transfer characteristic of lead–bismuth eutectic in a vertical circular tube are investigated. And the characteristics of natural circulation under inclination conditions are demonstrated at the same time. The research may provide a good basis for the safety analysis and validation of the thermal–hydraulic system analysis code in case of reactor transient accidents.