In the present study, the heat-transfer mode in the gap cooling was studied by analyzing boiling curves obtained in the experiments on transient cooling and rewetting of hot vertical surfaces in narrow gap channels. First, the experiments were carried out using annular channels with a narrow gap between inner and outer walls. The inner rod of the channel was made of stainless steel and the outer wall of glass tube for visualization. The initial temperature of the wall ranged from 500oC to 800oC, and the size of the gap ranged from 0.5 mm to 7.0 mm. Two-dimensional time-dependent heat conduction model was introduced in the study to calculate a boiling curve using the measured temperature history of the outer surface of the inner wall. The calculated boiling curves for the present gap cooling were compared with existing correlations to understand the heat-transfer characteristics of the gap cooling. The effects of the gap size on the gap cooling and rewetting process were also discussed. The results indicated that the heat transfer during the gap cooling was significantly limited by the countercurrent flow limitation (CCFL) in a narrow gap. Under the same initial temperature condition, if the size of the gap was larger, the rewetting of the inner wall took place earlier at a higher heat flux and the critical heat flux was also higher, which approached to the heat transfer characteristics of pool boiling when the gap size was larger than 4.0 mm. Finally, the contribution of axial heat conduction to the surface heat flux was discussed.