The nickel ferrite ultrafine powders with different grain sizes (8, 12, 20, 40 and 80 nm) have been synthesized. Mossbauer spectra of Fe-57 nucleus in the samples composed of the nickel ferrite ultrafine powder before and after the high pressure treatments have been measured. The superparamagnetic relaxation is markedly suppressed by high pressure. The intensity of the sextets in Mossbauer spectra increases with increasing the grain size or pressure. The effect of high pressure on Mossbauer spectra of the samples may involve not only the physical changes, but also the chemical or structural ones in the ultrafine particles during the high pressure treatment, such as the formation of the interfacial region between the ultrafine particles and the rearrangements of the atoms in the surface layers of the ultrafine particles.