We calculate the next-to-next-to-leading order
correction to the cross section for top quark pair production
in e+e- annihilation in the threshold region, resumming all
O [ ( alpha_s/beta )^n ( alpha_s^2, beta^2, alpha_s beta ) ]
terms of perturbation series. We find that the magnitude of
the NNLO correction is comparable to the size of the NLO
corrections.