We introduce a simple approach to synthesize high-quality Fe3O4 nanosheets at low temperature. By oxidizing pure Fe substrates in acidic solution on a hot plate maintained at 70 °C, we have grown the nanosheets in large-scale. The samples were characterized by SEM, XRD, TEM, micro-Raman, and VSM. Flowing O2 gas into the solution during oxidation enhances the growth rate of the nanosheets and thereby shortens the growth time from 24 h to 15 min. The magnetic property of the sample was also investigated. This technique can potentially be extended for large-scale synthesis of other metal-oxide nanosheets such as ZnO.