The current paper reports on a study of two-phase flow of liquid-vapour R134a in vertical circular channels with diameter of 1 mm using the Volume of Fluid (VOF) method. The purpose of these simulations was to develop a numerical flow regime map in order to identify the flow pattern boundaries. The vapour was injected using an annular (concentric) nozzle configuration. Two dimensional, axi-symmetry was assumed in order to save computational time and effort. Based on the numerical flow pattern maps developed in this study, four basic flow patterns were identified, namely: bubbly flow/confined bubble flow, slug flow, churn flow, annular flow. The present results were verified through comparison with the experimental flow visualization results and were shown to agree well. The study also showed that Computational Fluid Dynamics can be used to obtain a reliable two-phase flow pattern map.