Disclosed is a three-dimensional semiconductor memory device including a carbon-containing layer on a substrate, a plurality of electrode interlayer dielectric layers and a plurality of electrode layers that are alternately stacked on the carbon-containing layer, a cell vertical pattern that penetrates at least some of the electrode interlayer dielectric layers and the electrode layers, and a semiconductor pattern between the cell vertical pattern and the carbon-containing layer. The substrate includes a plurality of first grains. The semiconductor pattern includes a plurality of second grains. An average size of the second grains is less than an average size of the first grains.