According to example embodiments, a method of fabricating a semiconductor device includes: forming a preliminary stack structure including upper and lower preliminary stack structures by alternately stacking a plurality of interlayer insulating and sacrificial layers on a cell, first pad area, sacrificial area and second pad area of a substrate; removing an entire portion of the upper preliminary stack structure on the second pad area; forming a first mask defining openings over parts of the first and second pad areas; etching an etch depth corresponding to ones of the plurality of interlayer insulating and sacrificial layers through a remaining part of the preliminary stack structure exposed by the first mask; and repetitively performing a first staircase forming process that includes shrinking sides of the first mask and etching the etch depth through remaining parts of the plurality of interlayer insulating and sacrificial layers exposed by the shrunken first mask.