  Applying the narrow-width approximation (NWA), we first review the NLO QCD
predictions for the total decay rate of top quark considering two unstable
intermediate particles: the $W^+$-boson in the standard model (SM) of particle
physics and the charged Higgs boson $H^+$ in the generic type-I and II
two-Higgs-doublet models, i.e. $t\to b+W^+/H^+(\to \tau^+\nu_\tau)$. We then
estimate the errors arised from this approximation at leading-order
perturbation theory. Finally, we shall investigate the interference effects in
the factorization of production and decay parts of intermediate particles. We
will show that for nearly mass-degenerate states ($m_{H^+}\approx m_{W^+}$),
the correction due to the interference effect is considerable.
