In a method of manufacturing a non-volatile memory device, a tunnel insulating layer may be formed on a channel region of a substrate. A charge trapping layer including silicon nitride may be formed on the tunnel insulating layer to trap electrons from the channel region. A heat treatment may be performed using a first gas including nitrogen and a second gas including oxygen to remove defect sites in the charge trapping layer and to densify the charge trapping layer. A blocking layer may be formed on the heat-treated charge trapping layer, and a conductive layer may then formed on the blocking layer. The blocking layer, the conductive layer, the heat-treated charge trapping layer and the tunnel insulating layer may be patterned to form a gate structure on the channel region. Accordingly, data retention performance and/or reliability of a non-volatile memory device including the gate structure may be improved.