Well-defined Fe3O4 octahedrons have been readily realized in high yield via a simple and general hydrothermal synthesis technology free of any surfactant or template. Detailed investigations indicated that the octahedral Fe3O4 microcrystals are single crystal and were dominated by (110) surfaces. The influences of the amount of NaOH on the morphologies of the final products were investigated. The experimental results show that the high concentrations of NaOH play vital roles for the formation of octahdral Fe3O4 microcrystals. Besides, the magnetic property of the as-obtained Fe3O4 sample was evaluated as potential materials for electromagnetic device application on a vibrating sample magnetometer (VSM) at room temperature. The values of saturation magnetization and coercivity of octahedral Fe3O4 are approximately 95 emu g-1 and 197 Oe, respectively.