A method for forming a thin film using radicals generated by plasma may include generating radicals of a reactant precursor using plasma; forming a first thin film on a substrate by exposing the substrate to a mixture of the radicals of the reactant precursor and a source precursor; exposing the substrate to the source precursor; and forming a second thin film on the substrate by exposing the substrate to the mixture of the radicals of the reactant precursor and the source precursor. Since the substrate is exposed to the source precursor between the formation of the first thin film and the formation of the second thin film, the rate of deposition may be improved.