A method for forming a thin film by using an atomic layer deposition (ALD) method and a method for fabricating a capacitor using the same includes: supplying a source gas, a reaction gas, and a purge gas, then discontinuing the supply of the reaction gas and the source gas, followed by supplying and then discontinuing the supply of the reaction gas, wherein supplying the source gas, the reaction gas, and the purge gas, then discontinuing the supply of the reaction gas and the source gas, followed by supplying and then discontinuing the supply of the reaction gas constitutes a unit cycle, and repeating the unit cycle until a thin film having a desired thickness is deposited.