  D0 and CDF collaborations at the Fermilab Tevatron have searched for
non-standard-model single top-quark production and set upper limits on the
anomalous top quark flavor-changing neutral current (FCNC) couplings
$\kappa^g_{tc}/\Lambda$ and $\kappa^g_{tu}/\Lambda$ using the measurement of
total cross section calculated at the next-to-leading order (NLO) in QCD. In
this Letter, we report on the effect of anomalous FCNC couplings to various
decay branching ratios of the top quark, calculated at the NLO. This result is
not only mandatory for a consistent treatment of both the top quark production
and decay via FCNC couplings by D0 and CDF at the Tevatron but is also
important for the study of ATLAS and CMS sensitivity to these anomalous
couplings at the CERN LHC. We find that the NLO corrections to the partial
decay widths of the three decay channels $ t\to q + g$, $ t\to q + \gamma$ and
$ t\to q + Z$ are at the order of 10% in magnitude and modify their branching
ratios by about 20%, 0.4% and 2%, respectively, as compared to their leading
order predictions.
