  In this paper the time evolution of a two-level atom in the presence of
medium-assisted thermal field is explored through which, the formula of decay
rate of an excited atom is generalized in two aspects. The obtained formula
applies for the thermal electromagnetic field as well as the presence of
arbitrary arrangement of magneto-electric media. In order to be general with
respect to the material environment, the Green's function approach is used. It
is seen that the non-zero temperature contributes to the decay rate via an
additive term that is equal to the zero-temperature result multiplied by two
times of photon number at atomic transition frequency.
