Fire whirls often appear in large-scale urban and forest fires. They cause great damage and many victims, due to the involved tornado-like strong wind, radiation and tall swirling flames. Under special conditions, disastrous urban fires resulting from huge earthquakes may induce large scale fire whirls. However, despite much laboratory-scale research as reported in the literature, little is known about real large scale fire whirls. This paper presents a CFD simulation of fire whirls induced by wooden structures which burn in a wide urban area. Both stationary fire whirls and fire whirls moving across a barrier are simulated, for which the details of large scale fire whirls are examined. First, the physical characteristics of stationary fire whirls are analyzed. It is found that an asymmetrical wind can generate a fire whirl. The heat release rates of the burning structures and the total air mass entrained into the fire whirl both play important roles in the strength of fire whirls. The maximum flame length of the fire whirl is nearly 80 m at the heat release rate of 8 GW in a strongly burning area. In addition, moving fire whirls crossing over a barrier in an asymmetrical wind are simulated. The results show that the burning area moving downwind affects the location of fire whirls, and if a barrier exists in the propagating direction of a fire whirl, no fire whirls appear in a wide area downwind.