We report the synthesis of single-crystalline α-Fe2O3 nanoflakes by the oxidation reaction of water vapor through a gas-solid method. The samples are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), micro-Raman spectrometer, and transmission electron microscopy (TEM). As-synthesized nanoflakes have a pseudotriangle morphology: 20-50 nm in thickness, 0.5-1.5 μm in length and base-width. It is observed that vertically aligned arrays of leaf-like α-Fe2O3 grow at the verges of the iron foils. The possible mechanism is discussed to elucidate the formation of α-Fe2O3 nanostructures. The experimental results indicate that water vapor plays an important role in controlling the morphology of the final products.