A 3D memory device includes a substrate, a ridge-shaped stack, a memory layer, a channel layer and a capping layer. The ridge-shaped stack includes a plurality of conductive strips extending along a first direction and stacked on the substrate. The memory layer is stacked on a vertical sidewall of the ridge-shaped stack along a second direction that forms a non-straight with the first direction. The channel layer is stacked on the memory layer along the second direction and has a narrow sidewall having a long side extending along the first direction. The capping layer is stacked on the narrow sidewall along a third direction that forms a non-straight angle with the second direction.