The atomic structure of sputtered amorphous Tb-Fe1-xCox thin film systems was studied by X-ray scattering. The composition fraction x was varied systematically, and the basic atomic structures of Tb-Fe1-xCox films were similar as a whole. The interatomic distances in Tb-Fe1-xCox films annealed at 200°C, overall, tend to decrease when the Co content is increased. These distances seem to change discontinuously at a composition of around x=0.5. The coordination numbers of the corresponding atoms did not show much correlation with Co concentration except in the case of transition metal atoms around a transition metal atom. In this case, the coordination number increases after annealing, but this increment tends to decrease as the Co concentration increases in Tb-Fe1-xCox films. © 1991 The Japan Society of Applied Physics.