The improvement effect of replacing B with C or P on the formation ability, thermal stability and magnetic properties for melt-spun (Fe0.8Co0.2)85B14-xSi1Mx and (Fe0.8Co0.2)84B15-xSi1Mx amorphous alloy ribbons was examined because the base amorphous alloy exhibits ultrahigh saturation magnetization (MS) reaching 2.0 T The minor alloying by C or P causes an enhancement of its formation ability, a decrease in the onset temperature of the first-stage crystallization (Tx1) due to the precipitation of bcc FeCo phase and a slight increase in the second-stage crystallization temperature (Tx2) due to the decomposition of the residual amorphous phase, resulting in an extension of the temperature interval between Tx1 and Tx2. The annealing of their amorphous alloys causes a reduction of coercive force (HC) and an increase of effective permeability (µe) at 1 kHz, though the MS decreases slightly. The excellent soft magnetic properties of 1.98 T for MS, 8.0 A/m for HC and 5800 for µe were attained for (Fe0.8Co0.2)85B13Si1P1 amorphous ribbon with a thickness of 20 µm annealed for 500 s at 570 K. Their soft magnetic properties are promising for future application of the present amorphous alloys as a new high MS soft magnetic material for electric motors.