A method of manufacturing a semiconductor device may include forming a stack structure by alternately stacking sacrificial layers and interlayer insulating layers on a substrate, forming channel structures extending through the stack structure, forming openings extending through the stack structure, forming lateral openings by removing the sacrificial layers exposed by the openings, and forming gate electrodes in the lateral openings. Forming the gate electrodes may include supplying a source gas containing tungsten (W) wherein the source gas is heated to a first temperature and is supplied in a deposition apparatus at the first temperature, supplying a reactant gas containing hydrogen (H) subsequently to supplying the source gas, wherein the reactant gas is heated to a second temperature and is supplied in the deposition apparatus at the second temperature, and supplying a purge gas subsequently to supplying the reactant gas.