Atomic structures of a sputtered amorphous Ge 5 Sb 70 Te 25 thin film as a phase change optical recording material included in a rewritable optical disk have been studied by transmission electron microscopy (TEM) and nanobeam electron diffraction. Cross-sectional high-resolution TEM revealed that crystalline atomic cluster regions as small as 2 nm are formed in the assputtered amorphous Ge 5 Sb 70 Te 25 thin film. Atomic pair-distribution functions derived from a halo diffraction intensity analysis suggested that the as-sputtered Ge 5 Sb 70 Te 25 thin film including the crystalline clusters has an atomic configuration similar to that of the amorphous Sb structure.