An improved thermodynamic model is developed for the critical radius for droplet condensation on subcooled surfaces having a promoter coating layer. In addition to heat conduction through the droplet, this model takes into consideration thermal resistances of the coating layer, the liquid-vapor interface, and curvature depression of the equilibrium interface temperature, which are important in nano-size droplets. The critical radius for droplet condensation is obtained based on the availability criterion, and effects of contact angles, degree of subcooling, thickness and thermal conductivity of the coating layer, and saturated pressure on droplet nucleation radius are illustrated. Furthermore, the critical radius for condensation on a superhydrophilic subcooled surface without a promoter coating layer obtained from this new model recovers to the critical radius of droplet condensation in classical heterogeneous nucleation of droplets. (C) 2014 Elsevier Ltd. All rights reserved.