Abstract: Mn ferrite nanocrystals were synthesized by hydrothermal process and characterized by XRD, IR,SEM and VSM. The influences of hydrothermal temperature and time on the formation of Mn ferrite nanocrystals, structures and magnetic properties were studied. The results show that the hydrothermal samples synthesized at low temperature exhibit low crystallinity and purity, and poor magnetic properties. At 120 ℃, the samples exhibit the saturation magnetization of 15.34 emu/g, the remanence ratio of 0.08, and low coercivity of 81 Oe. When the hydrothermal temperature is fixed at 200 ℃, the samples are pure cubic MnFe₂O₄ nanocrystals with saturation magnetization of 51.49 emu/g,remanence ratio of 0.14 and high coercivity of 121 Oe.