Thin films of the Bi2(Sr,Ca)3Cu2Oxand YBa2Cu3O7−&dgr;system were annealed at 400 °C in a high‐density oxygen plasma and its effect on superconducting properties was investigated. After being annealed in the oxygen plasma, their superconducting transition temperatures decreased by about 10 K and 2 K, respectively, and thec‐axis lattice constants of these films were also found to decrease as a result of the annealing in the oxygen plasma. These results suggest that excessive oxygen incorporated into the films by the annealing in the oxygen plasma caused the excessive hole carriers, which deteriorated the superconducting transition temperatures of these films.