  The conventional thermal instability criterion can not be applied to the
advection-dominated accretion disks around black holes where the radiative
cooling is insufficient to balance the viscous heating. The surface density
change associated with the temperature perturbations, which was usually
neglected in deriving the conventional criterion, was recently shown to be much
significant in the advection-dominated disks. Considering both advection and
surface density change, I suggested a generalized thermal instability
criterion. By applying it to the optically thin and optically thick
advection-dominated disks, I found that the former one is thermally stable and
the latter one is thermally unstable against short wavelength perturbations,
which agrees well to those found recently by both analytic and quantitative
stability analyses.
