The forming mechanisms of the bulk metallic glasses (BMGs) were investigated from the viewpoints of thermodynamics and kinetics combined with the classical theory of nucleation and growth. The influences of the enthalpy change ΔHf, entropy change ΔSf, Gibbs free energy difference ΔG, melt viscosity η and cooling rate on the glass forming ability of bulk metallic glasses and their relationships were expatiated. In addition, some criteria of BMGs’ glass forming ability were summarized