  Nonlinear response of a Mott insulator to external electric field,
corresponding to dielectric breakdown phenomenon, is studied within of a
one-dimensional half-filled Hubbard model. It is shown that in the limit of
nearly spin polarized insulator the decay rate of the ground state into excited
holon-doublon pairs can be evaluated numerically as well to high accuracy
analytically. Results show that the threshold field depends on the charge gap
as $F_{th} \propto \Delta^{3/2}$. Numerical results on small systems indicate
on the persistence of a similar mechanism for the breakdown for decreasing
magnetization down to unpolarised system.
