The main purpose of the present work is to study the hydrodynamic behavior of high viscous oil-water flow within the horizontal ducts undergoing sudden expansion. Computational fluid dynamic (CFD) approach by means of Volume of Fluid (VOF), encoded in Ansys Fluent 16.2 was applied and experimental validation was conducted. Characteristics of two-phase flow, such as pressure gradient, spatial distribution of phases (Hold-up) and flow regimes have been extracted. Three pipe configurations for sudden expansion (21-30 mm, 30-40 mm, and 30-50 mm) were selected. The major flow patterns under investigation included core-annular (CAF) and transition from CAF to dispersed flows, with twelve cases considered for numerical runs. The presence of sudden expansion caused the oil core to become more eccentric and thicker far from singularity and without oil contact at the pipe wall during core-annular flow. This is consistent with the flow visualizations that were observed experimentally and discussed in details in the current paper. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.