In pool fires of some liquid fuels, the isothermal layer, or ＇＇hot zone,＇＇ is formed below the burning surface, and increases in thickness with time. Mechanisms of heat transfer in such medium-scale hot-zone-forming poor fires were studied. The distributions of the tank wall temperature and the heat transfer tate between the tank wall and the surroundings (flame, gas above the fuel surface, fuel, and surrounding air or water) in 0.559-m-diameter fuel oil and gasoline pool fires such as reported by Burgoyne et al. were calculated using a finite difference program. The following conclusions were obtained from the calculated results: the heat input and loss of the fuel through the tank wall become sometimes dominantly large, and are never negligible compared with the direct heat input from the flame to the fuel surface; when the tank is cooled with water, the heat loss from the fuel through the wall is so large that the hot-zone thickness decreases gradually; the rate of increase of the hot-zone thickness increases with increasing thermal conductivity and thickness of the tank wall; in pool fires of lower hot-zone temperature, the convective heat transfer from the wall to the fuel is dominantly large at the upper layer of the hot zone; in pool fires of comparatively high hot-zone temperature and low thermal conductivity of the tank wall, the downward current of the fuel near the tank wall must be caused due to the heat loss from the fuel to the tank wall throughout the hot zone.