Nanocrystalline Ni-Zn ferrite powders with spinel structure was prepared by mechanochemical route. In order to formation of this ferrite with composition of Ni 0.5 Zn 0.5 Fe 2 O 4 , it was used the stoichiometric mixture of NiO, ZnO and Fe 2 O 3 . Initial materials was milled in various times up to 60h. X-ray powder diffractometry (XRD), scanning electron microscopy (SEM), simultaneous thermal analysing (STA), Fourier transform infrared spectroscopy (FTIR), ac susceptometer and vibrating sample magnetometer (VSM) were carried out to characterize the structural, chemical and magnetic aspects of ferrite. The crystallite size of final product after 60 h of ball milling time was estimated to be 17nm by Williamson-Hall equation. In order to decrease crystal defects and lattice strain due to mechanical alloying and reduction of remained initial oxides, annealing treatment was done in different times and temperatures. The condition of 800°C and 2h was selected as a suitable condition for annealing and grain size increased to 45 nm after annealing at this condition. Ni 1-x Zn x Fe 2 O 4 (x=0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) was synthesized with previous conditions to study the effect of composition on the structural and magnetic properties. XRD results showed that lattice parameter of ferrite increased by increasing Zn content (x). Powder particles in all compositions had a spherical morphology and diameter size of around 100nm. Thermal analysis was used to determine the mechanism of formation of Ni-Zn ferrite. The formation of NiZnFe 2 O 4 phase appeared to involve two stages; development of Zn ferrite by diffusion of ZnO in Fe 2 O 3 followed by diffusion of NiO in Zn ferrite to form Ni–Zn ferrite. FTIR results showed that the ferrite has two specific bands in 420 and 590cm -1 which former depends on octahedral sublattice and the latter corresponds to tetrahedral sublattice. The tetrahedral band was shifted to higher frequencies but octahedral band had no displacement as Zn content increases. The saturation magnetization and coercivity of samples was obtained by plotting of hysteresis loop of B-H. The results showed that the saturation magnetization of milled ferrite was to be lower than that of annealed ferrite. And it is shown the x=0.7 and 0.9 samples exhibited the non-saturated magnetization, zero remanence and zero coercivity. These samples indicated also the superparamagnetism behavior. The measurement of ac susceptibility of different samples showed that the x=0.7 sample had a thermal peak in ~280K which shifted to higher temperatures as frequency increases. of this sample was agreed by Vogel-Fulcher law . Key Words Mechanical Alloying; Nanostructured; Magnetic Materials; Ni-Zn Ferrite