Currently existing expressions of glass forming ability (GFA) have been formulated based on the characteristic temperatures measured from heating of metallic glasses. However, these GFA expressions are not acceptable physically as GFA is defined from cooling of the molten liquid rather than heating of the solid glasses. In consideration of the relationship between the cooling and heating processes, we have derived a new GFA criterion, γ c= (3T x-2T g)/T l (T x: onset crystallization temperature; T g: glass transition temperature; T l. liquidus temperature). This criterion correlates very well to the critical cooling rate and agrees exceptionally well with the physically acceptable boundary condition. ; Department of Mechanical Engineering