  We consider the nonrotating isolated horizon as an inner boundary of a
four-dimensional asymptotically flat spacetime region. Due to the symmetry of
the isolated horizon, it turns out that the boundary degrees of freedom can be
described by a SO(1,1) BF theory with sources. This provides a new alternative
approach to the usual one using Chern-Simons theory to study the black hole
entropy. To count the microscopical degrees of freedom with the boundary BF
theory, the entropy of the isolated horizon can also be calculated in the
framework of loop quantum gravity. The leading-order contribution to the
entropy coincides with the Bekenstein-Hawking area law only for a particular
choice of the Barbero-Immirzi parameter, which is different from its value in
the usual approach using Chern-Simons theory. Moreover, the quantum correction
to the entropy formula is a constant term rather than a logarithmic term.
