This paper studies the mechanism for the formation of a slug flow in vertical gas-liquid two-phase flow. By analyzing void fraction waves and their instability, it is proved that the formation of a slug flow regime is due to the increase of void fraction waves, which causes the conglomeration of gas bubbles and the coalescence of bubble clusters in unstable bubbly flow. Experiments and analysis show that intense turbulence can restrain the formation of Taylor bubbles. Therefore, in a large diameter vertical pipe, a Taylor bubble can form under a condition of low continuous volume flux due to the action of void fraction waves. However, the coalescence effect of void fraction waves as it affects bubbles is suppressed in high continuous volume flux, and therefore, a slug flow regime cannot be observed in the evolution of flow patterns. Under a condition of high continuous volume flux (V-L = 0.15 m/s) described in the paper, the flow pattern evolution is from cap bubbly flow to cap churn flow, and then gradually to churn flow with the increase of void fraction. (C) 2002 Published by Elsevier Science Ltd.