  We study the quasinormal mode and stability of optical black holes in moving
dielectrics. The results show that the real part of complex frequencies is
inversely proportional to but the absolute value of an imaginary part is
proportional to a refractive index. We obtain the conditions for forming a
black hole horizon in moving dielectrics. Moreover, we investigate the
evolution behavior of optical black holes with respect to a refractive index at
a high overtone number and find that an optical black hole undergoes one phase
transition from an unstable mode to a stable one when the refractive index is
big enough.
