A method for cooling a battery pack, said battery pack comprising a U-type cooling structure in which a plurality of cells are arranged between a first cooling channel and a second cooling channel having a lower and an upper surfaces and are arranged to be spaced apart from each other by a predetermined distance, wherein: - a plurality of slits is formed in a portion of the lower surface of the first cooling channel not coupled with the cells and a plurality of slits is formed in a portion of the upper surface of the second cooling channel not coupled with the cells, - a coolant introduced through the first cooling channel flows between the cells through the plurality of slits and is then discharged through the slits of the second cooling channel in a direction opposite to the coolant introduction direction; - each channel have a constant sectional area in a length direction, and - a blowing fan is installed at an entrance of the first cooling channel, the method comprising the step of: (i) setting an exit flow rate of the coolant discharged from the second cooling channel, an entrance flow rate of the coolant introduced to the first cooling channel and a ratio of the exit flow rate to the entrance flow rate; (ii) adjusting the ratio of a sectional area of the second cooling channel to a sectional area of the first cooling channel to satisfy the following equation such that the temperature deviation of the cells is not greater than 5°C; and 1.65 × exit_flow_rate entrance_flow_rate 1.5 = C × ( sec tional_area_of_ sec ond_cooling_channel sec ond_area_of_first_channel wherein C is a constant ranging from 0.8 to 1.2; and (iii) introducing the coolant to the first cooling channel by means of the blowing fan, wherein the blowing fan gives a driving force for introducing the coolant into the first cooling channel. - a plurality of slits is formed in a portion of the lower surface of the first cooling channel not coupled with the cells and a plurality of slits is formed in a portion of the upper surface of the second cooling channel not coupled with the cells, - a coolant introduced through the first cooling channel flows between the cells through the plurality of slits and is then discharged through the slits of the second cooling channel in a direction opposite to the coolant introduction direction; - each channel have a constant sectional area in a length direction, and - a blowing fan is installed at an entrance of the first cooling channel, (i) setting an exit flow rate of the coolant discharged from the second cooling channel, an entrance flow rate of the coolant introduced to the first cooling channel and a ratio of the exit flow rate to the entrance flow rate; (ii) adjusting the ratio of a sectional area of the second cooling channel to a sectional area of the first cooling channel to satisfy the following equation such that the temperature deviation of the cells is not greater than 5°C; and 1.65 × exit_flow_rate entrance_flow_rate 1.5 = C × ( sec tional_area_of_ sec ond_cooling_channel sec ond_area_of_first_channel wherein C is a constant ranging from 0.8 to 1.2; and (iii) introducing the coolant to the first cooling channel by means of the blowing fan, wherein the blowing fan gives a driving force for introducing the coolant into the first cooling channel.