We study the dynamical behavior of the BTZ (Banados-Teitelboim-Zanelli
) black hole with the low-energy string effective action. The perturba
tion analysis around the BTZ black hole reveals a mixing between the d
ilaton and other fields. Introducing the new gauge (dilaton gauge), we
disentangle this mixing completely and obtain one decoupled dilaton e
quation. We obtain the decay rate Gamma of the BTZ black hole. [S0556-
2821(98)02520-X].