Spin-polarized small-angle neutron scattering (Pol-SANS) measurements for nano-crystalline Nd-Fe-B magnet have been performed in order to clarify the magnetic reversal behavior. Due to high transparency of neutron beam, magnetic and nuclear microstructures in the `bulk' can be observed by SANS. Obtained SANS patterns arising from magnetic domain changes were analyzed by Teubner-Stray model, phenomenological correlation length model, to quantify the periodicity, density and morphology of magnetic domains. The results indicate that magnetic reversal evolves with magnetic domains which have the similar size of the grains. However, even in nano-crystalline system, single domain grain reversal is not realized.