The Mn-Zn ferrite bodies with densities above 99.8% and fine grain sizes blow 5μm were obtained by sintering spinel ferrite powders prepared by hydrothermal method at relatively lower sintering temperature than that in the ordinary sintering process. The Mn-Zn ferrite bodies with grain sizes below 3μm have higher bending strength than the commercial Mn-Zn ferrite with grain sizes around 9 μm now used as present as a magnetic head material and relative permeabilities comparable to that of the commercial ones at a frequency range above 2 MHz. Therefore, the Mn-Zn ferrite with a high density and fine grain sizes is expected to be useful as the magnetic head for high density recording.