An etchant composition for etching a metal film and a method of forming a pattern, the etchant composition including an acid etching agent, the acid etching agent including an inorganic acid or an organic acid; an auxiliary oxidant, the auxiliary oxidant including hydrogen peroxide or an amine oxide compound; and an organic solvent, the organic solvent including a compound having an unshared electron pair, and having a dielectric constant of about 17 to about 80.