A new thermodynamic parameter PHSS (PHSS = ΔHC(ΔSσ/kB)(ΔS C/R)) is proposed, to describe and predict the glass forming ability (GFA) of Fe based alloys. Here ΔHC is the chemical enthalpy of mixing, ΔSσ is the mismatch entropy and ΔS C is the configurational entropy. The PHSS parameter incorporates enthalpy of mixing, size mismatch and the number of elements in the systems which influence GFA significantly. It was observed from rapid solidification processing (RSP) and mechanical alloying (MA) studies that, all alloys with PHSS in between -0.55 kJ/mol to -6.00 kJ/mol would form glass in the Fe-Zr-B system. MA and RSP experiments on multi-component Fe-Cr-Ni-Zr-B alloys indicated that PHSS is a better parameter to predict GFA of the system than PHS, a parameter used in earlier studies. It was also observed that bulk metallic glass (BMG) forming alloys can be observed in between a PHSS range of -3.00 kJ/mol to -6.00 kJ/mol, with the maximum thickness of the BMG increasing with decreasing PHSS within the above range. © 2012 Elsevier Ltd. All rights reserved.