A Co-based amorphous phase with glass transition and supercooled liquid region before crystallization was formed inCo70−xFexZr10B20andCo72−xFexZr8B20alloys containing more than 14 at &percent; Fe. The crystallization temperature(Tx)is 899 K for the Co–Zr–B alloys and remains unchanged in the Fe concentration range up to 20&percent;. The glass transition temperature(Tg)decreases with increasing Fe content, and the&Dgr;Tx(=Tx−Tg)increases from 25 K at 14&percent; Fe to 34 K at 21&percent; Fe. The amorphous alloys with glass transition crystallize with a single stage precipitation of bcc Fe(Co) andCo3ZrB2phases. The Co-rich amorphous alloys exhibit good soft magnetic properties, i.e., saturation magnetization of 0.58–0.83 T, low coercivity of 4.7–8.3 A/m, and high permeability of 5500–18 300 in the frequency range of1–103 kHzand low magnetostriction between−1.5×10−6and+10×10−6including zero. The success in synthesizing the soft magnetic amorphous alloys with high stability of supercooled liquid is promising for the future development of ferromagnetic Co-based bulk amorphous alloys. ©1998 American Institute of Physics.