In a recent paper, Gokcal et al. (2008) investigated the effects of viscosity on drift velocity in a horizontal pipe experimentally and theoretically. Experiments were conducted in a 50.8-mm (2-in.) ID horizontal pipe. A mechanistic model was developed. The results revealed that viscosity is a significant parameter for drift velocity which used to be a neglected factor in all previous literature. In this paper the effect of high viscosity in different pipe diameters on drift velocity is investigated using a Computational Fluid Dynamics (CFD) approach. New drift velocity data for 3-inch (76.2-mm) and 6-inch (152.4-mm) ID. pipes are presented. Experimental and simulation results are compared, and discussions on the effect of both diameter and liquid viscosity on the drift velocity for horizontal flow are made. The results of this work may improve the existing two-phase slug flow models for heavy oil.