The glass-forming ability (GFA) of an Fe 76 Si 9 B 10 P 5 bulk metallic glass (BMG) was evaluated thermodynamically with commercial software, Thermo-Calc, with Fe-base database, TCFE7 by utilizing its best ability to deal with equilibrium phases. The Fe 76 Si 9 B 10 P 5 BMG was selected because it is the simplest Fe-rich BMG belonging to Fe-metalloid type with the greatest sample dimensions. A possible reason for the presence of intermediate equilibrium phases to degrade GFA of the Fe 76 Si 9 B 10 P 5 was discussed. The results revealed that the Fe 76 Si 9 B 10 P 5 BMG is characterized by near eutectic composition in the Fe-rich Fe-Si-B-P quaternary system and by the simultaneous presence of Fe 3 P and absence of Fe 2 B phases in equilibrium at a range below the solidus-to the glass-transition temperatures. The analysis of Fe 76 (Si,B,P) 24 alloys for equilibrium phases revealed that the Fe 2 P phase can degrade GFA of the Fe