The thermal stability of the uniaxial anisotropies in Co86 Zr14 and Co91 Zr9 amorphous films prepared by rf sputtering are investigated through isothermal annealing in a static magnetic field. It is found that the stability is improved with increasing content of Zr. The relaxation curves have been fitted with calculated ones assuming the lognormal distribution of activation energy. The mean value of activation energy has been estimated at 2.5 eV for Co86 Zr14 and 1.7 eV for Co91 Zr9 films, respectively. The anisotropy increases with decreasing metal content, that can be explained by the precipitation caused by the nonmagnetic interaction in amorphous alloys.