Gd‐Ba‐Cu‐O superconducting thin films were oxidized by an ion doping technique using a microwave electron‐cyclotron‐resonance (ECR) plasma with AC(20kHz) substrate bias at a substrate temperature of 330 °C. Under conditions with the lowest emission intensity ratio of molecular oxygen ions(O2+) to oxygen atoms(O) observed by optical emission from O2plasma, superconducting films did not exhibit superconductivity after treatment. X‐ray diffraction indicated that bonded oxygen atoms were removed from the films by oxygen radical. Under conditions with high O2+/O ratio, the superconducting thin films were not remarkably changed. The effect of oxygen ion doping could be observed.