Further miniaturization and higher reliability of electronic devices are required for automobiles. The reliability of ferrites with high saturation magnetic flux-density, which is indispensable for automobiles, was investigated. A saturation magnetic flux-density of 450 mT at 100°C was obtained with substituting Ni for part of Mn in Fe-rich MnZn ferrite. However the deterioration of core loss was observed on the high temperature storage test at 150°C for 2000hr. The degradation degree of core loss increased on increasing the number of cation vacancy in sintered body. On optimizing the number of cation vacancy by controlling sintering atmosphere, MnNiZn ferrites with both high saturation magnetic flux-density and reliability at high temperature were obtained.