A set of burning experiments were conducted to investigate the effect of vertical shaft height on natural ventilation in urban road tunnel fires. Two special phenomena, plug-holing and turbulent boundary-layer separation were observed, both of which will influence the effect of smoke exhaust. When shaft height is relatively small, the boundary layer separation is significant and vortexes form in the upstream region inside the shaft, causing the backflow of gas mixture and preventing the throughflow of smoke. With the increasing of shaft height, the boundary layer separation becomes inconspicuous and the plug-holing occurs, leading to the ambient fresh air beneath smoke layer being exhausted directly, which will strongly decrease the smoke exhaust efficiency. Therefore, it is not the case that the higher the vertical shaft, the better the smoke exhaust effect, there exist a critical shaft height in which the boundary layer separation can be diminished to a large extent and overmuch entrainment of fresh air such as plug-holing can be avoided. In addition, the critical shaft height related to better effect can be determined by the new criterion of Ri' (Ri'(critical) = 1.4) proposed in this paper. (c) 2012 Elsevier Ltd. All rights reserved.