The effect of Si addition on the glass-forming ability and magnetic properties for the Fe₇₇Ga₃P_12−xC₄B₄Si_x and Fe₇₈Ga₂P_12−xC₄B₄Si_x glassy alloys was investigated. For the both alloys, the addition of 2.5 at%Si was found to be effective for the extension of the supercooled liquid region (ΔT_x) defined by the difference between the glass transition (T_g) and crystallization temperature (T_x). The ΔT_x value is 28 K for the Fe₇₇Ga₃P₁₂C₄B₄ alloy and increases to 48 K for the Fe₇₇Ga₃P_9.5C₄B₄Si_2.5 alloy. The effectiveness of Si addition can be interpreted in the framework of the three component rules for the formation of bulk glassy alloys and the stabilization of supercooled liquid. The three component rules are satisfied at a high degree of level by the addition of Si. The glassy single phase was obtained in the diameter range up to 2.5 mm for the Fe₇₇Ga₃P_9.5C₄B₄Si_2.5 glassy alloy and 2.0 mm for the Fe₇₈Ga₂P_9.5C₄B₄Si_2.5 glassy alloy. The glassy alloys exhibited high saturation magnetization (I_s) and good soft magnetic properties, i.e., high I_s of 1.36 and 1.40 T, and low coercive force (H_c) of 4.25 and 3.35 A/m, respectively. The high I_s and low H_c as well as high glass-forming ability of the Fe-based bulk glassy alloys are promising as a new type of soft magnetic bulk alloy.