A simple model of two-phase slug flow in inclined pipes is proposed. The model parameters are determined experimentally using five different size copper pipes at 5, 10, 15 deg inclinations on an air-water mixture at one atmosphere with up flow. The model predicts the total pressure gradient due to the sum of gravity and wall shear stresses. An investigation of the relationship between pressure gradient and pipe size results in an optimum pipe size at which the pressure gradient is minimum. A comparison between the simplified model predictions and experimental measurements shows a good agreement in the total pressure drop.