This paper has analyzed the effect of stack effect on natural ventilation with increasing shaft height in urban road tunnel fires by Large Eddy Simulation (LES). Results have shown that it is not the case that the higher the vertical shaft, the better the smoke exhausting effect. When shaft height increases to a certain value, the plug-holing effect occurs beneath smoke vent because of the excessive vertical inertia force induced by stack effect and the fresh air below smoke layer is drawn directly into the shaft, leading to the smoke exhausting process inefficiently. Based on the controlling inertia force acts on smoke movement, the exhausting process with different shaft heights can be divided into the horizontal inertia force control, the vertical inertia force control and the transition process between them. It comes to conclusion that the shaft can exhaust more smoke and avoid the plug-holing effect when it is at the critical plug-holing height under different heat release rates. So the critical plug-holing height is the optimal value for effective smoke exhausting. (C) 2013 Elsevier Ltd. All rights reserved.