To investigate the heat transfer characteristics of a condenser tube in a Passive Containment Cooling System (PCCS) of an advanced nuclear power plant, an experimental investigation has been conducted on the air-steam condensation over external surfaces of a vertical tube under natural convection conditions. Local temperature data and average heat transfer coefficients were measured on a test condenser of 40 mm in O.D. and 1.0 m in length while axial air concentrations and wall temperatures were kept almost uniform. The experiments covered the pressure ranging from 2 to 5 bar, the air mass fraction from 0.10 to 0.88, the wall subcooling from 19 to 70 K, and the Grashof number of an air-steam mixture varied from 1.36 . 10(10) up to 1.42 . 10(11) accordingly. In particular, the effect of the wall subcooling on the condensation heat transfer coefficient and the total heat removal rate was evaluated. From the comparison to Dehbi's tube data, it was found that the condensation heat transfer coefficients on a vertical tube were significantly influenced by the strength of natural convection of the air-steam mixture, which can be described by the Grashof number. To complement the inadequacy of physical bases in deriving previous correlations, a new empirical correlation for the condensation heat transfer coefficient was proposed by using a consolidated data of this study and Dehbi's test; the Nusselt number is correlated in terms of the Grashof number, Jakob number, and the air mass fraction. The suggested correlation was assessed by experimental data and its deviation was turned out to be less than 26%. (C) 2016 Elsevier Ltd. All rights reserved.