Nickel-zinc ferrites (Ni-Zn ferrites) nanocomposites were prepared by a solvothermal method and characterized by X-ray diffraction (XRD), scanning electronic microscope (SEM) and vibrating sample magnetometer (VSM). The effects of solvothermal parameters such as ratio of Ni2+ to Zn2+, the temperature and the time of solvothermal reaction on the magnetic properties and the microstructures of Ni-Zn ferrites were investigated. Results demonstrate that with raising of reaction time, the particles become bigger and more homogeneous, and the saturation magnetization of the Ni-Zn ferrites nanocomposites get higher; and the formation temperature of Ni-Zn ferrites spherical particles is 180 °C; raising Ni2+ concentration in the Ni-Zn ferrites could not change their morphologies. The saturation magnetization of the Ni-Zn ferrites increases with the increase of Ni2+ in the product; it will reach the highest when Ni2+ concentration is up to x=0.30 while it will be the lowest when Ni2+ concentration is down to x=0.20.