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In this study, we simulated the effects of the anisotropy field (Hk) dispersion of the grains (the main phases and the deterioration layers) for a NdFeB magnetic powder on Hc and squareness ratio in consideration of the magnetic properties of the grain boundaries and the deterioration layer of the grain surface using a micromagnetic simulator. The Hc decreases as increasing the coefficient variation of Hk (σHk/<Hk>; σHk: standard deviation, <Hk>: average) of the grain for the soft magnetic grain boundary, however, is independent of the σHk/<Hk> for the non-magnetic grain boundary. The squareness ratio is 1.0 up to a σHk/<Hk> of 25% for the soft magnetic grain boundary, however, decreases as the σHk/<Hk> increases for the non-magnetic grain boundary. Comparing the calculation and experimental values, when the grain boundary was non-magnetic and the σHk/<Hk> was 25 to 30%, the Hc and the squareness ratio were close to the experimental values.