One of the major problems in two-phase flow research is prediction of flow pattern at different configurations without requiring expensive experimental tests. Numerical modeling and simulation is a suitable and emerging approach for this purpose which has many benefits including saving in time and cost. In this study, for the first time, the effect of pipe rotation on the flow patterns of air-water two-phase flow in downward direction was numerically studied. For this reason, Eulerian-Eulerian multi-fluid approach was utilized in Ansys-Fluent software. At first, apparent conditions of each regime in various revolution were recorded from experimental tests and then the simulation was carried out to find if the results are compatible. It was shown that pipe rotation has important effects on the flow patterns map and shift transitions boundaries of slug and annular flow toward lower gas superficial velocity. Comparison of numerical results with experimental data show acceptable match for all regimes. Good agreement was observed for falling film, bubbly and slug regimes but the least agreement was observed for froth regime due to high turbulence and perturbation of flow. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.