[en] The nanocrystalline Zn_xMn_1-xFe₂O₄ (x = 0.1-0.9) powders were prepared by high-energy ball milling. Their structure and magnetic properties of Mn-Zn ferrites with various Zn content were investigated by means of X-ray diffractometer (XRD), scanning electron microscopy and magnetic property measurement system in this paper. The XRD results indicate that the formed particles with associated strains are ultrafine in diameter of about 18 nm. The lattice parameters increase proportionally with the increase of Mn content. The magnetic studies on the milled nanocrystalline Zn_xMn_1-xFe₂O₄ samples show that the ultrafine zinc ferrites exhibit net magnetization with a superparamagnetic behavior at room temperature. The clear understanding of this anomalous behaviour of MnZn ferrites in the nanocrystalline state needs to be studied further