  The nonclassicality of single photon-added thermal states in the thermal
channel is investigated by exploring the volume of the negative part of the
Wigner function. The Wigner functions become positive when the decay time
exceeds a threshold value $\gamma{t}_c$, which only depends on the effective
temperature or mean thermal photon number of the thermal channel, but not
depends on the effective temperature of the initial thermal state. This
phenomenon is similar with the case of single quantum excitation of classical
coherent states in thermal channel. Furthermore, we firstly demonstrate
$\gamma{t}_c$ is the same for arbitrary pure or mixed nonclassical optical
fields with zero population in vacuum state.
  PACS numbers: 42.50.Dv, 03.65.Yz, 05.40.Ca
