The decrease of initial relative permeability at 103-106Hz is observed in MgCuZn or MnMgCuZn ferrites. By measuring the temperature dependence of loss factor, the gradual deterioration of initial relative permeability is attributed to a relaxation phenomenon. The relaxation phenomena of MnMgCuZn ferrites occur in the condition of the stoichiometoric composition and the higher sintering temperatures. The activation energies of the relaxation phenomena are calculated to be 0.3-1.0eV. The relaxation times at infinite temperature are calculated to be 10-12-10-16 second. The initial relative permeability of MgCuZn or MnMgCuZn ferrites with the relaxation phenomena is higher than the value calculated from the resonance frequency and Snoek’s limit law.