In order to elucidate the physical processes of the formation of fire whirl, a series of laboratory experiments on three dimentional temperature profile of crib fire plume in cross wind were performed. From the experimental result, it became evident that a fire in cross wind produces a pair of vortex-like circulation in the downstream side of fire plume. In rare occasions, fire whirls were observed in the series of experiment of multiple fires at the central or leeward part of multiple fires under weak wind.