Atomic structures in the amorphous Fe 70 M 10 B 20 (M = Zr, Nb and Cr) alloys with different ΔT x values were studied by the anomalous X-ray scattering method. Their basic structural features resemble each other except a shape of a triangular prism as a local structural unit. The prism is slightly distorted in the amorphous Fe 70 M 10 B 20 (M = Zr and Nb) alloys because a part of Fe atoms at the vertices are replaced by the M elements. The larger the size difference between M and Fe is, the larger the ΔT x value becomes. This tendency and the observations of the crystallization process by the differential scanning calorimetry and X-ray diffraction measurements indicate us a close relationship between thermal stability of the amorphous state and the shape of the prism in the amorphous Fe 70 M 10 B 20 alloy.