Provided is a method of forming a thin film of a semiconductor device. The method includes forming a precursor layer on a surface of a substrate by supplying a precursor gas into a chamber, discharging the precursor gas remaining in the chamber to an outside of the chamber by supplying a purge gas into the chamber, supplying a reactant gas into the chamber, generating plasma based on the reactant gas, forming a thin film by a chemical reaction between plasma and the precursor layer and radiating extreme ultraviolet (EUV) light into the chamber, and discharging the reactant gas and the plasma remaining in the chamber by supplying a purge gas into the chamber.