In a sheet processing apparatus which is able to control to stack a sheet on at least one of a plurality of stacking units which stack printed sheets, when a first sheet having a small size is stacked on a first stacking unit, and a second sheet having the small size is designated to be stacked on a second stacking unit different from the first stacking unit, it is determined whether a sheet is stacked on a stacking unit adjacent to the second stacking unit, and a third stacking unit, that is different from the second stacking unit and is adjacent to a stacking unit on which a sheet is stacked, is selected as a stacking unit for the second sheet if it is determined that no sheet is stacked on the stacking unit adjacent to the second stacking unit.