Insituepitaxial growth of Bi2(Sr,Ca)3Cu2Oxfilms was performed by ion beam sputtering in atomic oxygen ambience at the substrate temperature of 640 °C. The films showed an epitaxial growth in which theaandbaxes were parallel to ⟨100⟩ MgO, and the superstructure according to the incommensurate modulation along thebaxis was also observed. The superconducting properties of the as‐grown films seemed to sensitively depend on the oxidation treatment during the cooling down process. The zero resistivity temperatureTc(R=0) of a 600‐A˚‐thick film cooled down in the same atomic oxygen density as the film growth ambience was 60 K, but it increased up to 80 K after a post‐deposition annealing at 500 °C for 1 h in air. In contrast, as‐grown films cooled down in insufficient oxidation ambience showed theTc(R=0) of 76 K without post‐deposition annealing. The control of the oxygen concentration is critical for the superconductivity of as‐grown films.