A memory device includes, from bottom to top, a substrate, a laminated layer and a stacked structure. Vertical channel pillars penetrate through the stacked structure and the laminated layer. First isolation structures are disposed aside the vertical channel pillars and penetrate through a lower part of the stacked structure. Second isolation structures are disposed over the first isolation structures and penetrate through an upper part of the stacked structure. Common source lines are disposed aside the vertical channel pillars and penetrate through the stacked structure and part of the laminated layer. From a top view, the common source lines extend in a first direction. Each of the first and second isolation structures has, in the first direction, two wide end portions respectively adjacent to two common source lines. The memory device may be applied in the process of manufacturing a 3D NAND flash memory with high capacity and high performance.