Abstract: The film condensation widely exists in the compact containment and the pressurizer of nuclear power plant. The Reynolds correlation in film condensation is the foundation of calculating heat transfer coefficient, and few studies focus on the difference between two Reynolds correlations based on mass and energy respectively. For outer tube natural convection film condensation of pure steam, by solving a set of simultaneous equations of the correlations of film condensation and different Reynolds correlations, the expressions of Reynolds number based on mass and energy in laminar and wavefree flow and in laminar and wavy flow were obtained, respectively. It is indicated that in laminar and wave-free flow, the definitions of Reynolds number based on mass Re_mass and that based on energy Re_energy are equivalent, and the heat transfer coefficients are the same. Whereas in laminar and wavy flow, Re_mass is smaller than Re_energy, and the maximum deviation of the average heat transfer coefficients based on these two definitions of Reynolds number is about 30.3%. Compared with experiment data, it is proven that the result based on Re_energy is more accurate in laminar and wavy flow.