An analytical model for laminar film condensation of pure vapor on the outer surface of an isothermal vertical tube is presented. Based on the assumptions employed in the Nusselt's analysis on a flat plate, the explicit expressions for the condensate film thickness as well as local and average Nusselt numbers are derived. The heat transfer from the liquid vapor interface is dealt with for two contrary situations: with and without the condensate subcooling. The deviation of the Nusselt number of a cylinder from that of a flat plate is presented in terms of the ratio of the liquid film thickness to the radius of a tube and the Jakob number. The variations of the Nusselt number with the tube radii are calculated from the derived models to investigate the curvature effects of a condensing surface. It is demonstrated that the effect of the curvature on the condensation heat transfer becomes important when the radius of a tube is of the similar order of magnitude as the liquid film thickness. (C) 2014 Elsevier Ltd. All rights reserved.