Magnetization, Curie temperature and perpendicular magnetic anisotropy have been investigated for Co100-xRx (R=Pr, Nd, Sm, Gd, Dy, Er) amorphous alloy films prepared by the evaporation method. The origin of the perpendicular magnetic anisotropy is discussed together with the results of Fe100-xRx (R=Pr, Nd, Sm, Gd, Dy, Er) amorphous alloy films. It is found that the perpendicular magnetic anisotropy consists of two terms which are proportional to the content and to the square of the content of rare earths. These terms can both be explained by a large one ion anisotropy of rare earths.