The formation of a single amorphous Fe-Cr-P phase was strongly related to the amount of phosphorus addition. The structure of the melt-spun Fe-8Cr-P alloys containing 16.5 at% or more phosphorus was confirmed to be amorphous by X-ray diffraction (XRD). However, high-resolution transmission electron microscopy (TEM) revealed that a microcrystalline bcc Fe phase was dispersed in the amorphous matrix of the Fe-8Cr-16.5P and Fe-8Cr-18P alloys. The corrosion behavior of Fe-Cr-P alloys is significantly sensitive to the presence of microcrystallites in the amorphous matrix. A thick corroded layer was formed on the Fe-8Cr-16.5P and Fe-8Cr-18P alloys by immersion in 9M H2SO4. A TEM image for the thick corroded layer of the Fe-8Cr-16.5P alloy showed that the microcrystalline bcc Fe phase was preferentially dissolved away in the solution and that the amorphous Fe-Cr-P phase remained on the surface of the alloy without dissolution. Accordingly, the thick corroded layer corresponds to the amorphous Fe-Cr-P phase which is covered with the passive film. The addition of 20 at% phosphorus is necessary to form an almost complete amorphous structure and to provide the high corrosion resistance even in 9M H2SO4.