We report a method for converting the surface of pure iron or stainless steel sheets (SUS304 or SUS410) to α-(Fe_xCr_1−x)₂O₃ (x = 0.80, 0.92 or 1) nanostructure arrays by performing microwave irradiation (1000 W, 2.45 GHz, single-mode) under an O₂ and Ar atmosphere. When the iron, SUS304 and SUS410 sheets were irradiated, large-scale vertically aligned α-Fe₂O₃, α-(Fe_0.80Cr_0.20)₂O₃, and α-(Fe_0.92Cr_0.08)₂O₃ nanostructures, respectively, were found covering these substrates. The synthesized nanostructures were single crystalline. The Fe/Cr atomic ratios of the substrate were almost preserved in the nanostructures synthesized by the microwave irradiation. The morphology and size of the nanostructures can also be controlled by varying the microwave power or the irradiation time. This method requires only a relatively short reaction time (7 min), and does not require the supply of metallic species from the vapour phase, a template, or a catalyst. On the basis of the results, we speculate a possible mechanism for the formation of these α-(Fe_xCr_1−x)₂O₃ nanostructures by microwave irradiation.