  It was reported that the NNLO correction to the total cross section near the
ttbar threshold in e+e- collisions is as large as the NLO correction to it. In
the thesis, we improve the convergence of the perturbative series of the total
cross section by choosing appropriate expansion parameters for both m_t and
alpha_s. With these two prescriptions, the perturbative convergence is much
improved. Correspondingly, the resulting ambiguity for the determination of m_t
is estimated to be 0.1GeV.
  We also study the effect of all types of anomalous top-quark EDM interactions
near the threshold for the first time. We show that one can disentangle these
EDMs by the dependence of CP-odd observables on the CM-energy and on the degree
of initial state polarization. The sensitivity to chromo-EDM at LC turns out to
be better than that at LC in the open top region, but to be worse than that at
LHC by a factor 1/10. The sensitivities to electroweak-EDMs near the threshold
are similar to those in the open top region at LC.
