In this work, condensation of steam inside a long inclined flattened channel was studied numerically. The simulated case is a flattened channel with a length of 10.7 m and a very high aspect ratio. The channel width and height are 0.0063 m and 0.214 m, respectively. The volume of fluid (VOF) multiphase flow formulation was used to present the governing equations. The flow field was assumed to be three-dimensional, unsteady and turbulent. Furthermore, the working fluid was water with constant properties at the specified saturation temperature. The effects of various parameters such as inclination angle, steam mass flux and saturation temperature on the condensation heat transfer coefficient, cooling wall temperature, and pressure drop along the channel were investigated. The present results showed very good agreement with the previous experimental work and available correlations. It was found that the increase in the inclination angle, steam mass flux and the decrease in the saturation temperature caused an increase in the heat transfer coefficient. The results also showed that the dominant mode of condensation was the drop-wise mode on the cooling wall. Furthermore, in some cases, complete condensation was observed, which caused sub-cooling in the condensate river and backflow from the exit region of the channel. (C) 2019 Elsevier Ltd. All rights reserved.