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Fracture toughness obtained with standard procedures of analysis is a function of the constraint condition attending a crack tip. In recent times, the width of the stretch zone that develops prior to the initiation of ductile fracture has been of interest. It is instructive to observe the effect of variation of constraint on stretch zone formation. The stretch zone width can be employed as an effective marker for identifying fracture toughness from resistance curves. From this premise, it is important to study the variation of fracture toughness determined thus with variation of constraint.In this work, the effect of constraint on stretch zone width and fracture toughness determined using it have been examined for two pressure vessel and piping steels. This has been contrasted with the toughness metrics obtained through standard procedure, and major deviations noted. Of particular significance is the apparent independence of stretch zone fracture toughness with constraint.