A substrate processing method capable of stably performing atomic layer etching without damaging a process chamber is provided. The substrate processing method comprises providing a substrate including a target layer in a chamber, forming a first plasma in the chamber by using a first gas containing chlorine to first reform the target layer, forming a second plasma in the chamber by using a second gas containing oxygen to second reform the first reformed target layer, providing a precursor into the chamber to react the second reformed target layer with the precursor, and removing at least a portion of the target layer by repeating forming the first plasma, forming the second plasma, and providing the precursor.