A memory device includes a first electrode line layer including a plurality of first electrode lines extending on a substrate in a first direction and being spaced apart from each other, a second electrode line layer including a plurality of second electrode lines extending on the first electrode line layer in a second direction that is different from the first direction and being spaced apart from each other, and a memory cell layer including a plurality of first memory cells located at a plurality of intersections between the plurality of first electrode lines and the plurality of second electrode lines, each first memory cell including a selection device layer, an intermediate electrode and a variable resistance layer that are sequentially stacked. A side surface of the variable resistance layer is perpendicular to a top surface of the substrate or inclined to be gradually wider toward an upper portion of the variable resistance layer. The first memory cell has a side surface slope so as to have a width gradually decreasing toward its upper portion.