Inventive concepts provide semiconductor memory devices and methods of fabricating the same. A stack structure and vertical channel structures are provided on a substrate. The stack structure includes insulating layers and gate electrodes alternately and repeatedly stacked on the substrate. A first vertical channel pattern is disposed in a lower portion of each vertical channel structure. A gate oxide layer is formed on a sidewall of the first vertical channel pattern. A recess region is formed in the substrate between the vertical channel structures. A buffer oxide layer is formed in the recess region. An oxidation inhibiting layer is provided in the substrate to surround the recess region. The oxidation inhibiting layer is in contact with the buffer oxide layer and inhibits growth of the buffer oxide layer.