Using the improved double distribution function (DDF) thermal Lattice Boltzmann method (LBM), dropwise condensation of dry saturated vapor at a cold spot on vertical hydrophobic flat plates under gravity effects are simulated numerically for the first time. Dynamic behaviors of periodic formation of 3D condensing droplets on a 2D cold spot at a constant subcooled temperature, and their subsequent growth, and movement on hydrophobic surfaces are studied. The size of detached droplet on hydrophobic surfaces with different wettabilities under various gravity forces are simulated, and the results are found in good agreement with existing analytical predictions. The induced transient velocity and temperature fields and the variation of dropwise condensation heat flux with time at the cold spot on hydrophobic surfaces are analyzed. It is found that both the period of droplet condensation and its maximum heat transfer rates decrease as the contact angle of the surface is increased. (C) 2013 Elsevier Ltd. All rights reserved.