Amorphous strontium hexaferrite (SrFe12O19) nanoparticles produced by co-precipitation were treated by a hydrothermal synthesis. The strontium in the hexaferrite leached from the structure during the hydrothermal treatment, producing uniform, submicron hexagonal plate-like hematite particles. The residual Sr to Fe ratio in the hematite particles was about 1at%, which is lower than that of previously synthesized submicron plate-like hematite with inorganic ions as structure-directing agents. The plate-like hematite particles maintained their shape even after the hydrogen reduction, producing plate-like α-Fe particles having a saturation magnetization of 197emu/g and a coercivity of 162Oe.