  We consider the entropy problem of AdS_3 black holes using the conformal
field theory at the horizon. We observe that the supersymmetry is enhanced at
the horizon of massless AdS_3 black hole. This allows us to determine the
vacuum of the modular invariant conformal field theory to be the NS-ground
state (which corresponds to AdS_3 spacetime). This is smoothly related to the
R-ground state (corresponding to massless black hole) by a spectral flow, which
can be understood as a superconformal transformation.
