A method for forming a nitrogen-containing oxide thin film by using plasma enhanced atomic layer deposition is provided. In the method, the nitrogen-containing oxide thin film is deposited by supplying a metal source compound and oxygen gas into a reactor in a cyclic fashion with sequential alternating pulses of the metal source compound and the oxygen gas, wherein the oxygen gas is activated into plasma in synchronization of the pulsing thereof, and a nitrogen source gas is further sequentially pulsed into the reactor and activated into plasma over the substrate in synchronization with the pulsing thereof. According to the method, a dense nitrogen-containing oxide thin film can be deposited at a high rate, and a trace of nitrogen atoms can be incorporated in situ into the nitrogen-containing oxide thin film, thereby increasing the breakdown voltage of the film.