A lithium air battery comprising a plurality of unit cells which have different diameters, each unit cell comprises: electrodes including: a disc-shaped positive electrode having a first air flow path passing through the positive electrode in a vertical direction of the lithium air battery and one or more electrolyte flow paths on the positive electrode in a horizontal or vertical direction of the lithium air battery; and an negative electrode having a second air flow path passing through the negative electrode in the vertical direction to coincide with the first air flow path; and a separator disposed between the positive electrode and the negative electrode. The unit cells are stacked in the vertical direction within a stack cell tank such that a diffusion layer is disposed between the respective unit cells. A lowermost unit cell has the greatest diameter and diameters of the remaining unit cells, which are sequentially stacked on a lowermost unit cell, gradually decreases vertically in an upward direction so that the unit cells have a stepped structure.