The present invention relates to a kind of Mn-Zn ferrite, belonging to the field of oxide magnetic material technology, its main components are: Fe2O3 49-58 mole%, MnO 21-31 mole % and ZnO 15-26 mole%, and the additive component is added, they can be made into ring magnetic core whose mean grain size is 20-80 micrometer, outer diameter is 25 millimeter, inner diameter is 15 millimeter and height is 5 millimeter. In the frequency range of 1K-150 KHz its magnetic conductivity can be up to high level more than 12000, when it is used for making transformer, the number of turns can be reduced, said transformer can be miniaturized, specially, the transformer with high-frequency characteristics can be obtained in the digital transmission system of ISDN and ADSL, etc. and low volume distribution can be implemented, and the signal can be accurately transmitted.