A method for forming an oxide film by plasma-assisted cyclic processing, includes: (i) supplying a precursor to a reaction space wherein a substrate is placed; (ii) applying a first RF power to the reaction space for a first period of time without supplying a precursor; and (iii) applying a second RF power to the reaction space for a second period of time without supplying the precursor, wherein the first RF power is lower than the second RF power, and/or the first period of time is shorter than the second period of time.