The present invention provides a method of manufacturing a semiconductor device, which comprises the steps of: forming a buffer layer formed of a dual-layer structure of a buffer oxide film and a buffer nitride film on a semiconductor substrate formed with a certain lower structure; forming source/drain by performing an ion injection process after forming the buffer layer; defining a gate hole by etching the buffer layer after forming the source/drain; forming a gate oxide film on the defined gate hole; forming a gate material to bury the defined gate hole; forming a T-shape gate electrode through a process of etching the gate material using the buffer nitride film as an etching stop film; and forming a contact hole after forming an inter-layer dielectric on a resulting structure formed with the T-shape gate electrode.