Ti-Al-Nb alloys in the composition range studied in the present investigation were found to undergo stress-induced martensitic transformation (SIMT) in α2 + β heat-treated (two-phase) condition under tensile loading. The effect of different volume fractions of primary α2 on trigger stress for SIM transformation in these alloys was investigated. The trigger stress required for SIM transformation decreased initially when the volume fraction of primary α2 was increased from 0 to 5 pct. However, subsequently, the trigger stress was found to increase with further increase in volume fraction of primary α2 from 5 to 40 pct in all three alloys. An increase in the volume fraction of primary α2 is achieved by changing the solution treatment temperatures, which also results in a change in the β composition and β grain size. The observed variation of trigger stress with the volume fraction of primary α2 from 5 to 40 pct was then explained on the basis of the effect of composition and grain size of the β phase on trigger stress required for stress-induced transformation of β to martensite. © The Minerals, Metals & Materials Society and ASM International 2008.