Abstract: Based on analysis of flow mechanism,a hydrodynamic model of slug flow in vertical pipes was developed considering the variable thickness of liquid film.Effects of film thickness on results were dis- cussed and the model was evaluated with experimental data available from the literature,It was found that if the variation of thickness was neglected,and Taylor bubble was considered as a cylindrical bubble,the length of Taylor bubble would be under predicted.The simplification can also result in an over prediction of pressure gradient,and the deviation will increase as gas superficial velocity increases for a fixed liquid superficial ve- locity.The improved model takes into account the flow characteristic of liquid film,and can predict the data obtained from different sources with a fairly well accuracy.