  In this letter, we elaborate on the SL(2,Z) action on three dimensional
conformal field theories with U(1) symmetry introduced by Witten, by trying to
give an explicit verification of the claim regarding holographic dual of the S
operation in AdS/CFT correspondence. A consistency check with the recently
proposed prescription on boundary condition of bulk fields when we deform the
boundary CFT in a non-standard manner is also discussed.
