In ultrafiltration processes, injecting gas to create a gas-liquid two-phase crossflow operation can significantly increase permeate flux and, moreover, can improve the membrane rejection characteristics. It has been shown that controlled pulse injection to generate slug flow is more advantageous than uncontrolled gas sparging, especially when the gas flow rate is low. The slug size and frequency affect the performance of ultrafiltration, and there exits an optimal slug size and frequency to achieve high permeate flux. Previous studies have been based on the analysis of the experimental data and mass-transfer correlations. In this work, an attempt is made to model the slug flow ultrafiltration process using the volume of fluid (VOF) method with the aim of understanding and quantifying the details of the permeate flux enhancement resulting from gas sparging. For this numerical study, the commercial CFD package, FLUENT, is used. The first part of the model uses the VOF method to calculate the shape and velocity of the slug, the velocity distribution and the distribution of local wall shear stress in the membrane tube (neglecting the wall permeation effect). The second part links the local wall shear stress to the local mass-transfer coefficient that is then used to predict the permeate flux. In order to validate the model, experimental data reported in the literature over a wide range of gas and liquid velocities, slug frequencies, and transmembrane pressures are compared with the CFD predictions. Good agreement is obtained between theory and experiment.