We succeeded in the exfoliation of Bi2SrTa2O9 to a mono-nanosheet for the first time. The thickness of the nanosheet obtained by AFM measurement was about 1.3 nm. The nanosheet itself showed visible blue luminescence under excitation at 285 nm at room temperature. The luminescence property of the nanosheet was largely sensitive to the change in the surface environment such as adsorption of H+ and/or OH-.