Provided is a method of manufacturing a semiconductor device in which properties of photoresist through a lithography process are changed to form a dummy structure, and the structure is applied to a process of forming a gate electrode. The method includes the steps of: forming a buffer layer on the top of a semiconductor substrate; applying an inorganic photoresist on the buffer layer, and forming a photoresist pattern using a lithography process; thermally treating the photoresist pattern using a predetermined gas; uniformly depositing an insulating layer on the thermally treated structure, and etching the deposited layer by the deposited thickness in order to expose the thermally treated photoresist pattern; depositing an insulating layer on the etched structure, and etching the deposited insulating layer to expose the thermally treated photoresist pattern; removing the exposed photoresist pattern using an etching process; forming a gate oxide layer in the portion in which the photoresist pattern is removed; and forming a gate electrode on the gate oxide layer. Accordingly, in forming a structure for manufacturing a nano-sized device, the properties of the layer formed by a lithography process are improved through thermal treatment, and thus the structure used to manufacture various devices can be easily formed.