Internal fire whirls induced by a pool fire in a square vertical shaft were studied by experiments and numerical simulations. The burning behaviour of two pool fires in the vertical shaft and in open air was compared. The gap width of the rig is a key factor in onsetting fire whirls. Air flow field in the vertical square shaft of different gap widths were studied experimentally with nine tests. A fire whirl would not be onsetted when the gap is too narrow nor too wide. Whirling flame is not clearly observed near to the bottom of the vertical shaft when the gap width was small.