Magnetite film was successfully prepared on the mild steel surface in alkaline environment by hydrothermal electrolysis treatment. The principle of hydrothermal electrolysis is applying current directly to electrolyte solution at hydrothermal conditions. The anodizing of mild steel was conducted at temperatures of 100–200 °C and applied current densities of 0.1–0.4 A/cm2 for 5–60 min reaction time with sodium hydroxide (NaOH) as an electrolyte solution (5–15%). Magnetite film on the mild steel surface was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and cyclic voltammetry. The patterns of XRD showed that magnetite was formed dominantly in the surface of mild steel with black color. Cyclic voltammetry analysis showed that the magnetite films produced on the mild steel surface could prevent or minimize the corrosion on its surface.