The dried gel of SrFe_ 12O_ 19, 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 SrFeO_ 2.5 with an orthorthombic cell and Fe_2O_3 with a tetragonal cell as well as possibl α-Fe_2O_3. 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-Fe_2O_3 and strontium M-type hexaferrite. The method of distinguishing γ-Fe_2O_3 with a spinel structure from Fe_2O_3 with tetragonal cells by using powder XRD method was proposed. Fe_2O_3 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-Fe_2O_3 as an additional phase appears in the calcinations is the cationic vacancy of strontium M-type hexaferrite, SrFe_ 12-x_xO_ 19 (0≤x≤0.5). The dried gel of SrFe 12O 19 prepared by citrate approach was was investigated by infrared spectroscopy (IR), thermogravimetric analysis (TG), differential scanning calorimetry (DSC), X-ray diffraction EDS), and transmission electron microscopy (TEM). The thermal instability and the thermal decomposition of low-temperature strontium M-type hexaferrite crystallized at about 600 ° C were confirmed by the first time by XRD method. The decomposition of the low-temperature strontium M-type hexaferrite took place at about 688.6 ° C determined by DSC investigation. The low-temperature strontium M-type hexaferrite nanoparticles were decomposed into SrFeO_ 2.5 with an orthorthombic cell and Fe_2O_3 with a tetragonal cell as well as possibl α-Fe_2O_3. The agglomerated particles with sizes less than 200 nm obtained at 800 ° C were plesiomorphous to strontium M-type hexaferrite. The thermally stable strontium M-type hexaferrite nanoparticles with sizes less than 100 nm could take place at 900 ° C. Up to 1000 ° C., the phase transformation to form strontium M-type hexaferrite was ended, the calcinations with the sizes more than 1 μm were composed of α-Fe_2O_3 and strontium M-type hexaferrite . The method of distinguishing γ-Fe_2O_3 with a spinel structure from Fe_2O_3 with tetragonal cells by using powder XRD method was proposed. Fe_2O_3 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-Fe_2O_3 as an additional phase appears in the calcinations is the cationic vacancy of strontium M-type hexaferrite, SrFe_12-x__xO_19 (0≤x≤0.5).