  It was claimed by the author that black holes can be considered as
topological insulators. They both have boundary modes and those boundary modes
can be described by an effective BF theory. In this paper, we analyze the
boundary modes on the horizon of black holes with the methods developed for
topological insulator. Firstly the BTZ black hole is analysed, and the results
are compatible with the previous works. Then we generalize those results to
Kerr black holes. Some new results are obtained: dimensionless right- and
left-temperature can be defined and have well behaviors both in Schwarzschild
limit $a\rightarrow 0$ and extremal limit $a\rightarrow M$. Upon the Kerr/CFT
correspondence, we can associate a central charge $c=12 M r_+$ with an
arbitrary Kerr black hole if a dual CFT exists. We can identify the microstates
of the Kerr black hole with the quantum states of this scalar field. From this
identification we can count the number of microstates of the Kerr black hole
and give the Bekenstein-Hawking area law for the entropy.
