Fe-based glassy alloys with high boron concentrations in (Fe, Co)–(Nd, Dy)–B system were found to exhibit a large supercooled liquid region (ΔT_x) and good soft magnetic properties. The Fe₆₂Co_9.5Nd₃Dy_0.5B₂₅ glassy alloy with ΔT_x of 56 K exhibits high saturation magnetization (I_s) of 1.41 T, low coercive force (H_c) of 2.6 A/m and large permeability (μ_e) at 1 kHz of 12000. The Fe₆₂Co_9.5Nd₃Dy_0.5B₂₅ glassy alloy rods were produced in the diameter range up to 0.75 mm by copper mold casting. The substitution of 2 at% elements TM (TM=Nb, Ta, Mo and W) for Fe and Co, remarkably increases the ΔT_x and T_g⁄T_l, leading to an increase in the glass-formation ability (GFA) for Fe_60.3Co_9.2Nd₃Dy_0.5B₂₅TM₂. The ΔT_x and T_g⁄T_l increase from 56 to 87 K and 0.56 to 0.57, respectively, and the glassy alloy rods were produced in the diameter range up to 1.2 mm. The saturation magnetization I_s decreases slightly, while the coercive force H_c remains almost unchanged by the addition of 2 at% TM elements.