  We calculate the complete next-to-leading order QCD corrections to the
three-body decay of gluino into top-pair associated with a lightest neutralino
in the minimal supersymmetric standard model. We obtain that the LO and NLO QCD
corrected decay widths of $\tilde{g} \to t \bar{t} \tilde{\chi}_1^0$ at the
${\rm SPS 6}$ benchmark point are $0.1490 GeV$ and 0.1069 GeV respectively, and
the relative correction is -28.2%. We investigate the dependence of the QCD
correction to $\tilde{g} \to t \bar{t} \tilde{\chi}_1^0$ on $\tan\beta$ and the
masses of gluino, scalar top quarks and the lightest neutralino around the
${\rm SPS6}$ benchmark point, separately. We find that the NLO QCD corrections
suppress the LO decay width, and the absolute relative correction can exceed
30% in some parameter space. Therefore, the QCD corrections to the three-body
decay $\tilde{g} \to t\bar{t}\tilde{\chi}^0_1$ should be taken into account for
the precise experimental measurement at future colliders. Moreover, we study
the distributions of the top-pair invariant mass ($M_{t\bar t}$) and the
missing energy ($E^{miss}$), and find that the line shapes of the LO
distributions of $M_{t\bar t}$ and $E^{miss}$ are not obviously distorted by
the NLO QCD corrections.
