NiZn ferrite bulk materials have been fabricated using the conventional ceramic sintering method with a changing grain sizes from ～2.2 µm to ～13.5 µm. From the measurements of the complex permeability, it is suggested that the permeability is dominated only by the spin rotation at the mono-domain state and both domain wall motion and spin rotation contribute at the multi-domain state. At the mono-domain state, the core loss has been significantly decreased. The measurement result for the loss angle indicates that the low loss state can be maintained up to the higher magnetic field with the smaller grain size in spite of the mono-domain state. This study suggests the possibility of the magnetic coupling between the grains providing a new knowledge about the effect of the microstructure on the magnetic domain structure and the magnetization in NiZn ferrite.