  A method to extract nucleon-nucleon (NN) potentials from the Bethe-Salpeter
amplitude in lattice QCD is presented. It is applied to the two nucleons on the
lattice with quenched QCD simulations. By disentangling the mixing between the
S-state and the D-state, we obtain central and tensor potentials in the leading
order of the velocity expansion of the non-local NN potential. The spatial
structure, the quark mass dependence and the velocity dependence of the NN
potential are analyzed. Preliminary result in (2+1)-flavor QCD simulations is
also shown.
