An amorphous phase with a wide supercooled liquid region before crystallization was formed in Fe56Co7Ni7Zr10-xMxB20 (M=Nb or Ta, x=0–10 at. %) alloys by melt spinning. The glass transition temperature (Tg) and crystallization temperature (Tx) increase by the dissolution of 2% M and the degree of the increase is larger for Tx, leading to maximum ΔTx(=Tx-Tg) of 85 K at 2% Nb and 87 K at 2% Ta which are larger by about 20 K than the largest value for newly developed Fe–(Al, Ga)–(P,C,B,Si) amorphous alloys. The crystallization of the Nb-containing alloys occurs through two stages of amorphous (Am)→Am′+α-Fe+γ-Fe+Fe76Nb6B18 →α-Fe+γ-Fe+Fe76Nb6B18+Fe2Zr in the range less than about 6% Nb and Am→Am′+γ-Fe→γ-Fe+Co3Nb2B5+Ni8Nb in the range above 8% Nb. The change in the crystallization process with Nb content seems to reflect the ea- sy precipitation of γ-Fe by the increase in the number of Fe–Nb pairs with weaker bonding nature as compared with the Fe–Zr pairs. The best soft magnetic properties were obtained at 2% Nb or 8% Ta. The saturation magnetization, coercive force, effective permeability at 1 kHz, and saturated magnetostriction in the annealed state for 300 s at 800 K are, respectively, 0.96 T, 2.0 A/m, 19 100, and 10×10-6 for the 10% Zr alloy, 0.75 T, 1.1 A/m, 25 000, and 12×10-6 for the 2% Nb alloy, and 0.85 T, 1.5 A/m 17 400, and 14×10-6 for the 8% Ta alloy. The Curie temperature is 531 K for the 2% Nb alloy and 538 K for the 8% Ta alloy. The success in synthesizing the new amorphous alloys with the wide supercooled liquid region and good soft magnetic properties is promising for future development as soft magnetic bulk amorphous alloys. © 1998 American Institute of Physics.