The effect of a drainage layer on the saturation of coalescing filters was evaluated experimentally. The effect of the pore size of a drainage layer on the saturation was demonstrated using the capillary theory. The experimental results showed that the filter without a drainage layer began to drain in the liquid film forming stage, where the flow resistance of the liquid in the channels increased significantly, resulting in the increases in both the saturation of the filter and the thickness of the liquid film. The pressure drop, saturation and liquid film thickness profiles varied after assembling a drainage layer outside of the coalescing layer. There was an adjustment stage of the liquid film in the filtration process of the filter with a non-wettable drainage layer, which can be verified by the evolution of the thickness of the liquid film predicted using theoretical calculations. The amount of the liquid increased gradually between the coalescing layer and the drainage layer at this stage, resulting in a significant increase in the saturation of the coalescing layer. As assembling a wettable drainage layer, a pseudo-steady state appeared, where the pressure drop and penetration were steady while the saturation of each layer increased. After the pressure drop increased and became steady again, the drainage led to a drastic increase in the saturation of the coalescing layer. At steady state of all the filters, the drainage rate and loading rate were almost the same. Furthermore, the capillary theory can be used to analyze the variation on the saturation affected by the pore sizes of a wettable drainage layer. Smaller pore size led to stronger capillarity, resulting in more liquid absorbed into the drainage layer and greater saturation of the coalescing layer. (C) 2017 Published by Elsevier Ltd.