The thermal stability of the supercooled liquid, glass forming ability (GFA) and magnetic properties were examined for amorphous Fe72-xAl5Ga2P11C6B4Six, Fe72Al5Ga2P11-xC6B4Six, and Fe72Al5Ga2P11C6-xB4Six alloys. The increases in the thermal stability and GFA and the improvement of the soft magnetic properties were recognized in the replacements of P by 1 to 2 at. % Si and of C by 1 at. % Si. The supercooled liquid region ΔTx defined by the difference between Tx and Tg increases from 53 K for Fe72Al5Ga2P11C6B4 to 58 K for Fe72Al5Ga2P11C5B4Si1. The maximum thickness for glass formation (tmax) by copper mold casting also increases from 1 mm for the Fe–Al–Ga–P–C–B alloy to 3 mm for the Fe72Al5Ga2P10C6B4Si1 alloy. The increases in ΔTx and tmax are presumably because of the increase in the degree of the satisfaction of the three empirical rules for glass formation. The soft magnetic properties are also improved by the replacement of 1% Si through the increase in the squareness ratio of B–H loop (Br/Bs) and the decrease in coercive force (Hc). The best soft magnetic properties for the bulk amorphous alloys are 1.14 T for Bs, 1.5 A/m for Hc, 0.45 for Br/Bs and 594 K for Tc for the Fe72Al5Ga2P10C6B4Si1 alloy. The success of forming the Fe based ferromagnetic bulk amorphous alloys of 3 mm in thickness is promising for future development as a new type of magnetic material. © 1997 American Institute of Physics.