In this study, computational fluid dynamics (CFD) simulation is presented to study the behaviours of horizontal annular flow in small pipes. A 1.5 m-long horizontal pipe with three different diameters of 8.8 mm, 15.1mm, and 26.3 mm were analyzed for different air and water superficial velocities. Transient two phase annular flow regime behaviours were investigated by using ANSYS Fluent platform 17.2. For multiphase flow model, volume of fluid method (VOF) for two-phase flow modeling was used and coupled with RNG k-e turbulence model to predict fully annular flow structures at high flow rates. The in-situ film thickness and pressure drop data with three different diameters and superficial air velocities were examined. The numerical results agree well with the experimental data that studied the measurements of film thickness and pressure drop and also agree with the common models that predicted the transition between two-phase flow regimes in horizontal pipe at high flow rates. This study will be very helpful in understanding multiphase fluid flow in natural gas pipelines.