Two-phase flow is said as the flow of two phases simultaneously in a pipe. Two-phase flow occurs in both production and injection wellbores. In injection wellbores, the two-phase moves downward and so it is known as downward flow. The complexity of two-phase flow starts due to the present of flow patterns in the pipe. The flow patterns vary due to the density and viscosity differences between the phases which contribute to the difference in velocity of both phases. The geometry of the well is another factor which donates to the difference in flow pattern. The simultaneous flow of these two-phases creates a pressure drop which is caused by the loss due to friction, acceleration and elevation. The friction lose is due to the friction between both the phases besides the friction between the phase and the pipe wall. This study is aimed at calculating the friction factor of two-phase flow in injection wellbores based on different flow patterns. The calculation of two-phase flow has been developed by various scholars and a few mechanistic models been published. Based on the available mechanistic models, Hasan and Kabir model has been chosen with the reason of accuracy and continuity. © 2011 IEEE.