A method of manufacturing a semiconductor device includes forming insulation layers and sacrificial layers that are alternately and repeatedly stacked on top of each other a substrate, forming a vertical hole that penetrates the insulation layers and the sacrificial layers, and forming a vertical channel structure in the vertical hole. The forming the vertical channel structure includes forming a blocking insulation layer, a charge storage layer, a tunnel insulation layer, and a semiconductor pattern. The forming the blocking insulation layer includes forming a first oxidation target layer, oxidizing the first oxidation target layer to form a first sub-blocking layer, and forming a second sub-blocking layer. The first sub-blocking layer is formed between the second sub-blocking layer and an inner sidewall of the vertical hole.