Amorphous Co100-xTbx alloy thin films with the composition of x=7–60 at. % were prepared by dc magnetron sputtering at various powers and argon pressures then annealing in vacuum. Effects of the composition, sputtering power, argon pressure, and annealing temperature on the parallel and normal to the film plane magnetic properties have been investigated. The analysis of transmission electron microscopy diffraction patterns and magnetic measurement data indicate that an amorphous film with isotropic magnetic properties can be produced after low temperature annealing. The maximum observed perpendicular coercivity was as high as 6000 Oe for the as-deposited Co–Tb amorphous films. A nearly magnetically isotropic amorphous Co–Tb film with in-plane coercivity of about 2080 Oe was obtained after annealing. © 1998 American Institute of Physics.