We have measured the temperature dependence of magnetization and coercive force of amorphous Fe5Co70Si15B10 ribbons in the intermediate temperature region below the Curie point. The M(T) curves manifest a plateau in the temperature region between 220 and 300 C in relatively low fields of technical saturation, i.e., M(T) is nearly constant. The plateau of M(T) curves from d.c. measurements disappears in higher fields. The M(T) curves have been analyzed by the use of a modified molecular field theory. They suggest the coexistence of a ferrimagnetic phase with a ferromagnetic phase. The ferrimagnetic phase has a TN at 513K, and TC at 640K, which is experimentally indistinguishable from TC of the ferromagnetic phase. On the Hc(T) curves, the coercivity decreases slightly with increasing temperature at first, then increases sharply up to a maximum value near 563K.