Devising a new mechanistic method to predict gas-liquid interface shape in horizontal pipes is concerned in this article. An experiment was conducted to find the pressure gradients of air-water flow through a 1-in. pipe diameter. Comparing results of model with some experimental data available in the literature demonstrates that the model provides quite better predictions than existed models do. This model also predicts flow regime transition from stratified to annular flow better than Apparent Rough Surface and Modified Apparent Rough Surface models for both 1- and 2-in. pipe diameters. The model also leads to reliable predictions of wetted wall fraction experimental data. Although one parameter of new model was evaluated based on air-water flow pressure loss experimental data for 1 in. pipe, it was considerably successful to predict pressure drop, liquid holdup, stratified-annular transition and wetted wall fraction for other gas-liquid systems and pipe diameters. (c) 2014 American Institute of Chemical Engineers AIChE J, 61: 1043-1053, 2015