An etchant composition includes an inorganic acid, a siloxane compound, an ammonium compound, and a solvent, wherein the siloxane compound is represented by General Formula (I): A method of fabricating an integrated circuit device includes forming a structure on a substrate, the structure having a surface on which an oxide film and a nitride film are exposed; and selectively removing the nitride film from the oxide film and the nitride film by bringing the etchant composition into contact with the structure.