Differential scanning calorimeter in non-isothermal mode has been used to study the effect of addition of Hf, in place of Zr, on the glass forming ability and thermal stability of metallic glasses of composition Fe_57.2Co_30.8Zr_{7- x}Hf_xB₄Cu₁ (x=3, 5, and 7). An increase in the content of Hf results in an increase in the primary crystallization temperature and melting temperature. The activation energy of glass transition and primary crystallization was determined using commonly used thermo-analytical models. Relatively higher activation energy was seen in sample with lowest Hf content both for the glass transition and primary crystallization processes. An increase in Hf content showed a decline in thermal stability and glass forming ability assessed through fragility index,temperature range of super cooled liquid region, and other frequently used theoretical parameters. The role of Hf in this alloy system has been rationalized on the basis of these observations.