This paper presents a numerical study of dense-phase horizontal pneumatic conveying of powders by means of Computational Fluid Dynamics (CFD). The validity of the numerical model is first verified by comparing predicted pressure gradients with both published laboratory and industrially measured data. Then, effects of gas superficial velocity, solids loading ratio and pipe diameter on the flow regime transition and dune formation in horizontal pipes are investigated. Results show that dune, stratified and suspension flow regimes can be successfully reproduced. The transition of the suspension flow to the dune flow is determined mainly by the proportion of gas and solid phases. With increasing pipe diameter, the dune flow changes to the stratified flow. Also, flow characteristics are analyzed with respect to different flow regimes. It reveals that the formation of the dune flow is due to the intensive interaction between suspension and accumulation zones through a contact zone. (C) 2019 Elsevier B.V. All rights reserved.