  The one-loop correction and two-loop contribution to black-body radiation
(BBR) shift are restudied. The S-matrix approach and nonrelativistic quantum
electrodynamics (NRQED) are adopted in finite temperature case. The
relativistic correction to one-loop BBR-shift has a $(Z\alpha)^{2}\alpha
T^2/m$-order contribution. In the two-loop case, the pure thermal (real) photon
part is too tiny to be detected; while the corrections induced by the thermal
and virtual mixing diagram are at $(Z\alpha)^{2}\alpha^2 T^2/m$ order. We
calculate the relativistic correction to one-loop BBR-shift in the ground state
of hydrogen and ionized helium, which is larger than the leading term. As the
leading term is proportional to $T^4/Z^4$. We estimate these higher-order
corrections may be larger than the leading term, when the system is a highly
ionized (large $Z$) or a cold (small $T$) one.
