A memory device includes first conductive lines extending on a substrate along a first direction; second conductive lines extending on the first conductive lines along a second direction intersecting with the first direction; and memory cell structures, which are at intersections between the first conductive lines and the second conductive lines and connected to the first conductive lines and the second conductive lines, each of the memory cell structures including a first electrode layer, a second electrode layer, and a resistive memory layer between the first electrode layer and the second electrode layer. A first sidewall of each of the resistive memory layers is sloped and has a horizontal width that decreases in a direction away from the substrate, and a second sidewall of each of the resistive memory layer adjacent to the first sidewall is sloped and has a horizontal width that increases in a direction away from the substrate.