The temperature-dependent saturation magnetization curves of amorphous $Fe_{80-x}Co_xB_{20}$ (8 ${le}$ x ${le}$ 40, the step of x is 8) alloys were measured using a vibrating sample magnetometer from 77 K up to 1000 K. Curie temperature and the Bloch coefficient were estimated from the saturation magnetization curves. The low temperature dependence of magnetization is in good agreement with Bloch relation, $M_s(T)$=$M_s(O)$ (1-$BT^{3/2}$ - ${cdots}.$). The spin wave stiffness constant, tne range of the exchange interaction, and the probable atomic spin were calculated from the saturation magnetization values.