  The dijet invariant mass distributions from the hadronic decay of unpolarized
top quark ($t\to b W^+$ followed by $W^+\to u \bar{d}$) are calculated,
including the next-to-leading order QCD radiative corrections. We treat the top
decay in the complex mass scheme due to the existence of the intermediate state
W boson. Our analytical expressions are also available in different dimensional
regularization schemes and $\gamma_5$ strategies. Finally, in order to
construct the jets, we use different jet algorithms to compare their influences
on our results. The obtained dijet mass distributions from the top quark decay
are useful to distinguish these dijets from those produced via other sources
and to clarify the issue about the recent CDF Collaborations' $Wjj$ anomaly.
