The main purpose of this study is experimental and numerical void fraction measurement in two-phase flow inside a vertical pipe by using gamma-ray. Three types of flow regimes including homogenous, stratified and annular were modeled in a vertical pipe by using polyethylene phantoms. These three flow regimes are basis regimes in two-phase flow and the other flow regimes are incorporated of these patterns. For all three modeled flow regimes all transmitted and scattered gamma rays in all directions were measured by setting a gamma ray source and detector around the pipe. Numerical modeling was done by using MCNP code to improve the accuracy and validation of experimental results. Finally, innovative correlations to predict the void fraction in two-phase flow in a vertical pipe was presented. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.