In this paper, an experimental study is reported of variable property convection heat transfer of molten salts through a double-pipe steam generator. Molten salts flows upward in the vertical passage of annular cross-section and water evaporates in the inner tube. The heat transfer coefficients of molten salts have been calculated under buoyancy-influenced conditions with Reynolds numbers ranging from 2,500 to 12,000. A buoyancy parameter which combines Grashof number, Reynolds number and Prandtl number in a particular manner designed to characterize the strength of buoyancy influences. When the buoyancy parameter is less than 10−6, the buoyancy is less effective and a correlation equation with a maximum error of 20% is attained. However, when the buoyancy parameter is greater than 10−6, the variable properties of molten salts have a significant effect on heat transfer due to the high temperature difference between inner wall and molten salts. The results presented in this paper show, that molten salts flowing upward in a passage of annular cross-section having a chilled inner surface and an adiabatic outer one, the heat transfer behavior is generally similar to that air flowing downward in a passage of annular cross-section with a heated inner wall. But there are certain important differences. The variable properties of molten salts are stronger and the buoyancy force will be impaired under the viscous drag. Enhancement of heat transfer occurs with the onset of buoyancy influences, but more gradually and at a somewhat higher value of buoyancy parameter than the air case. Base on the experimental results, an equation correlated satisfactorily in terms of Nusselt number ratio (mixed to forced) and buoyancy parameter is proposed.