Investigation of natural ventilation using shafts in tunnels has been receiving more attentions, however, analyses on how shaft dimension and amount influence the natural ventilation performance have rarely been addressed. For the sake of fire protection and construction of tunnels, the influence of vertical shaft arrangement on natural ventilation performance during tunnel fires is investigated numerically by Large Eddy Simulation. The smoke flow characteristics in the tunnel and shaft under the combined function of longitudinal wind and stack effect of shaft are analyzed. Results show that both plug-holing and boundary layer separation will influence the natural ventilation performance. As a whole, the total mass flow rate of smoke exhausted by shafts increases with the shaft amount under a given total area of shafts. The case with maximum shafts for natural ventilation can gain the best ventilation performance in spite of the longitudinal wind. The case with the largest longitudinal wind velocity will gain the minimum total mass flow rate of smoke exhausted in spite of the shaft amount, due to the fact that a very obvious boundary layer separation occurs in the shaft. It is suggested that the cross-section of one shaft opening in the actual engineering design is oversize in general, which is not in favor of exhausting smoke. The influence of natural ventilation on smoke backflow and a special phenomenon, smoke bifurcation are also investigated. (C) 2014 Elsevier Ltd. All rights reserved.