A set of condensation experiments in the pure steam and steam-air conditions have been conducted to evaluate the heat removal capacity of a vertically longitudinal finned tube in natural convection situation. The bulk temperature, wall temperature and temperature near the tube wall were measured in detail. The result of the temperature measurement demonstrated that there was an air rich region near the longitudinal finned tube while gas stratification phenomenon was not found in the mainstream. Condensation heat transfer coefficient and heat transfer rate have been obtained for air mass fraction ranging from 0.07 to 0.95, total pressure ranging from 0.2 MPa to 0.7 MPa. A smooth tube was also tested under the same condition as a reference. At pure steam case, the longitudinal finned tube played a positive effect on the condensation heat transfer, showing a growth of the condensation heat transfer coefficient by a factor of 3.86-4.89. At the steam-air case, the longitudinal finned tube generated restrictions on the condensation heat transfer process when air mass fraction was below 75%. However, the longitudinal finned tube enhanced the heat transfer process for the air mass fraction in excess of 75%. (C) 2015 Elsevier Ltd. All rights reserved.