In this paper a mechanistic one-dimensional approach to the prediction of the transition from stratified to annular two-phase flow in horizontal pipes is presented. Such transition is deemed to occur when the liquid film wets the whole of the pipe circumference. This is determined from a consideration of the effects of the interface curvature, which is modeled here by a double circle geometric configuration incorporating a correlation for the calculation of the wetted angle. The model is cast in the framework of the two-fluid model and incorporated in a numerical procedure. The model also accounts for local droplet entrainment and deposition between the film and the gas core. With the model, it is possible to predict the growth and spread of the liquid film around the circumference along the pipe in a dynamic manner; transition to annular flow eventually occurs in a seamless manner when the film wets the whole of the circumference. The results are evaluated by comparing the numerical prediction of the transition from stratified to annular flow with an experimentally determined flow pattern map found in the literature. The comparison shows satisfactory agreement with experimental data.