Slug flow is an intermittent two-phase flow pattern characterized by gas and liquid flowing in the stratified form with unstable waves that grow to the upper part of the pipe. The purpose of this study is to determine the tube length for the flow development of a slug pattern using Computational Fluid Dynamics (CFD) technique. A horizontal geometry pipe with 7 m of length and 74 mm of internal diameter was build, where the inlet was set as a stratified flow. The superficial velocities applied were 10 m/s for air and 1 m/s for water. Results of water volume fraction profile show that the mathematical models were capable of predicting the slug flow behavior and the flow is fully developed in L/D ~= 68. In conclusion, the 7 m pipe is suitable to study this flow pattern.