The hydrothermal synthesis of Ni-Zn ferrite from simulated wastewater containing Ni2+ and Zn2+ ions has been studied. The influence of co-precipitation order, the existence of Na+ in suspension, the hydrothermal reaction time and temperature on the composition, morphology and saturation magnetization (sigma(s)) of the hydrothermal products is reported. Adding the simulated wastewater to the NaOH solution can prevent the formation of alpha-Fe2O3 in the Ni-Zn ferrite. Increasing the hydrothermal reaction time improved the magnetization of the Ni-Zn ferrite, while the influence of temperature, stirring intensity and Na+ in suspension on the hydrothermal formation of ferrite were not obvious. Thermodynamic calculation indicated that under hydrothermal conditions (180-240degreesC), the order of chemical stability is as follows: NiFe2O4 > Fe2O3 > Na2Fe2O4. The high Gibbs formation energy of Na2Fe2O4 prevented the incorporation of Na+ into the ferrite lattice.