The dried gel of SrFe12O19, prepared by citrate approach, was investigated by means of infrared spectroscopy (IR), thermogravimetric analysis(TG), differential scanning calorimetry(DSC), X-ray diffraction(XRD) techniques, energy dispersive spectroscopy(EDS), and transmission electron microscopy(TEM). The thermal instability and the thermal decomposition of low-temperature strontium M-type hexaferrite crystallized at about 600 ℃ were confirmed for the first time by XRD method. The decomposition of the low-temperature strontium M-type hexaferrite took place at about 688.6 ℃ determined by DSC investigation. The low-temperature strontium M-type hexaferrite nanoparticles were decomposed into SrFeO2.5 with an orthorthombic cell and Fe2O3 with a tetragonal cell as well as possibl α-Fe2O3. The agglomerated particles with sizes less than 200 nm obtained at 800 ℃ were plesiomorphous to strontium M-type hexaferrite. The thermally stable strontium M-type hexaferrite nanoparticles with sizes less than 100nm could take place at 900 ℃. Up to 1000 ℃, the phase transformation to form strontium M-type hexaferrite was ended, the calcinations with the sizes more than 1μm were composed of α-Fe2O3 and strontium M-type hexaferrite. The method of distinguishing γ-Fe2O3 with a spinel structure from Fe2O3 with tetragonal cells by using powder XRD method was proposed. Fe2O3 with tetragonal cells to be crystallized before the crystallization of thermally stable strontium M-type hexaferrite was confirmed for the first time. The reason why α-Fe2O3 as an additional phase appears in the calcinations is the cationic vacancy of strontium M-type hexaferrite, SrFe12-x()xO19 (0≤x≤0.5).