A set of burning experiments with n-heptane pool fire were conducted to investigate the air entrainment mode with natural ventilation using shafts in road tunnel fires. One criterion was proposed to determine the critical shaft height of the plug-holing occurrence in our previous work whereas this study quantificationally analyzes the effect of plug-holing on the smoke exhaust efficiency. The disturbance of smoke exhausting on the smoke-air interface, which causes different amounts of fresh air exhausted directly and indirectly, is investigated. Results show that using vertical shafts to discharge smoke leads to a strong mixing process between the smoke layer and the fresh air. Consequently, about 2/3 of the smoke exhausting rate of the shaft is air. Moreover, some of the entrained air mixes into the smoke layer downstream the shaft, resulting, in a small reduction amount of the spilling smoke, which also proves an inefficient smoke exhausting process. Hence, for the sake of improving effectiveness of natural ventilation, some significant improvements are needed in practice. (C) 2012 Elsevier Ltd. All rights reserved.