  We performed one-dimensional hydrodynamic simulations with detailed cooling,
heating and chemical processes to examine the thermal stability of shocked gas
in cold neutral medium (CNM) and molecular clouds. We find that both CNM and
molecular clouds can be thermally unstable in the cooling layer behind the
shock wave. The characteristic wavelength of the thermal instability ranges
from 10^-5 pc to 0.1 pc in the CNM, and from 10^-7 pc to 0.1 pc in the
molecular clouds. This coincides with the size of observed tiny scale
structures in the CNM and molecular clouds, indicating that the thermal
instability in the shocked gas could be a formation mechanism of these tiny
structures in the interstellar medium. We have also calculated the e-folding
number of the thermal instability to estimate the amplification of the density
fluctuation in the shocked gas. Density perturbations in the CNM grow by a
factor of exp(5)~150, whereas the perturbations in the molecular clouds grow
only by a factor of a few behind a high Mach number shock. The amplification
factor is larger at lower densities and higher velocities. Formation of very
small-scale structures by thermal instability in shocked gas is more effective
in lower densities.
