  We study the dynamical behavior of the BTZ (Banados-Teitelboim-Zanelli) black
hole with the low-energy string effective action. The perturbation analysis
around the BTZ black hole reveals a mixing between the dilaton and other
fields. Introducing the new gauge (dilaton gauge), we disentangle this mixing
completely and obtain one decoupled dilaton equation. We obtain the decay rate
$\Gamma$ of BTZ black hole.
