We investigated the formation mechanism of magnesium doped hercynite during reaction sintering process at 1400 ºC. A two-step synthesis procedure based on combining of spinel and ferrous chloride was considered here. Firstly, magnesium aluminate spinel was prepared by sol-gel method, using Al and Mg chlorides as raw materials. Then, the powder particles were dispersed in an aqueous solution of ferrous chloride in 1.5:1 molar ratio and the solution was stirred at 60 °C. After solvents removal, the thermal properties of a sample of the dried specimens were studied, using simultaneous thermal analysis (STA) method. Afterwards, the specimens were calcined at 400, 800, 1100 and 1400 ºC for 1, 3 and 6 hrs, under air atmosphere. X-ray diffraction (XRD) analysis, field emission scanning electron microscope (FESEM) and X-ray fluorescence spectroscopy (XRF) were used for characterization of the specimens. Study of the obtained results leads us to conclude that hercynite was formed thorough a diffusion-based mechanism, when heated to high enough temperatures. After heating for a prolonged period (6 hrs.) at 1400 °C, hercynite decomposes into its constituent oxides and also a new crystal phase appears. This new phase was identified as FeAlO3. The modified Scherrer, Williamson-Hall and Rietveld methods are also used to estimate the crystallite size of the as-synthesized magnesium doped hercynite.