MgFe 2 O 4 nanocrystallites were synthesized by a sol–gel/combustion method. The thermal decomposition of the precursor and subsequent formation of MgFe 2 O 4 were investigated by TG–DSC, FT-IR, XRD and TEM. A well-crystallized MgFe 2 O 4 powder was obtained after annealing in air at 500°C for 2h. The average particle size of the ferrite calcined at 500°C is ∼10nm, with a narrow size distribution. The particle size increased dramatically with the increase of calcining temperature. The saturation magnetizations of the nanocrystalline MgFe 2 O 4 calcined at 500–900°C are 9.9 and 30.6emug −1 , respectively.