A 3D memory device comprising a memory unit block, a first stair-step structure, a second stair-step structure, a first conductive strip and a second conductive strip is provided. The memory unit block comprises a first stacked structure comprising a first semiconductor strip and a second stacked structure comprising a second semiconductor strip. The first stair-step structure is disposed on one side of the memory unit block. The second stair-step structure is disposed on an opposite side of the memory unit block. The first conductive strip electrically coupled to the first semiconductor strip via the first stair-step structure. The second conductive strip electrically coupled to the second semiconductor strip via the second stair-step structure.