The present study deals with the detection and correlation of the stress relaxation phenomena that are induced in NiCuZn ferrites, as an effect of an additional annealing process that takes place after sintering. The effect of annealing on the magnetic permeability and losses of NiCuZn ferrite materials is investigated and the microstructural changes of the samples are examined by transmission electron microscopy. The ferrite specimens were synthesized by the conventional ceramic method. Sintering temperatures between 1025 and 1100 A degrees C were applied and the sintered samples were further annealed at temperatures 50 A degrees C lower than the sintering ones. Results revealed an improvement of the power losses at low frequencies (50 kHz), while at the high frequency range (1 MHz) the relative loss factors increased. These results were in agreement with the detected microstructural changes in the polycrystalline microstructure that occurred due to the annealing process. The relation between the annealed microstructure, the specific resistivity and the magnetic losses of the NiCuZn ferrites specimens is discussed. It appears that the stress relaxation mechanism induced by annealing is attributed to the elimination of grain dislocations, which justifies the decrease of the specific resistivity of the specimens. The stress relaxation contributes to a reduction of the low frequency hysteresis losses while the decrease of the resistivity to an increase of the high frequency eddy current losses.