  The cross section of $e^+e^- \to b \bar{b} u \bar{d} \mu^- \bar{\nu}_\mu$
process with a complete set of tree diagrams, 232 diagrams in the unitary
gauge, was calculated at the energy range of $\sqrt{s}$ = 340 - 500 GeV by
using GRACE system. A main contribution to the cross section comes from
$t\bar{t}$ production, where $t$ and $\bar{t}$ decay into $bu\bar{d}$ and
$\bar{b} \mu^- \bar{\nu}_{\mu}$, respectively. It was found that the
interference between the diagrams with $t\bar{t}$ production and those with
single-$t$ through $ WW $ pair production amounts to 10% at the $t \bar{t}$
threshold energy region. In the energy region above twice of the top quark
mass, more than 95% of the cross section comes from the $t\bar{t}$ diagrams.
