Nano-crystalline NiFe2O4 (NF) powder was prepared by a sol-gel auto-combustion method. Its crystal structure, morphology, particle size and magnetic properties were investigated and the formation process of nano-crystalline NF was elucidated. The as-dried Ni-Fe precursor (Ni/Fe=0.5) was calcined in air at 200-800°C for 4 hours. γ-Fe2O3 powder was formed at about 210°C by the self-combustion of the gel, and was maintained at 200-400°C. Single-phase NF sample was obtained by being calcined at 600-800°C. The NF particles were of nano-crystalline with crystallite diameters of 68.2-94.3 nm which were determined by the use of Scherrer equation for XRD peak broadening and the observation of TEM images. The sample calcined at 200°C had a magnetization value of 44 emu/g, which was lower than that of bulk γ-Fe2O3 (70 emu/g) because of a lot of vacancies at octahedral sites. Magnetization was increased gradually with increasing calcination temperature, and reached to the maximum value of 46 emu/g for the samples calcined at 700 and 800°C, which was in agreement with the magnetization of the bulk NF sample. Coercivity was decreased with increasing calcination temperature.