A series of structure transformations were observed for thick, sputter‐deposited amorphous Sm2Co17 alloys heat treated between 500°C and 900°C for one hour. The amorphous phase crystallized at approximately 500°C to give a CaCu5‐type structure with a c/a ratio of 0.86, which is the highest value ever observed for these alloys. With increasing annealing temperature, the c parameter decreased and the a parameter increased resulting in a continuous reduction of the c/a ratio to a value of 0.84 at 800°C, which is the same as that of the Th2 Zn 17‐type structure. These structure transformations and lattice parameter variations are attributed to a progressive change with annealing in the number and degree of order of Sm‐atom/Co‐atom pair substitutions. Based on a correlation found between the c/a ratio and composition for the CaCu5‐type Sm‐Co alloys, the metastable phase crystallized from the amorphous state at 500°C has an approximate formula of Sm2Co24 to Sm2Co26.