  Within the framework of perturbative resummation scheme of Pisarski and
Braaten, the decay- or damping-rate of a moving heavy quark (muon) to leading
order in weak coupling in hot QCD (QED) is examined. Although, as is well
known, the conventionally-defined damping rate diverges logarithmically at the
infrared limit, shown is that no such divergence appears in the physically
measurable decay rate. The cancellation occurs between the contribution from
the \lq\lq real'' decay diagram and the contribution from the diagrams with
\lq\lq thermal radiative correction''.
