The effect of β grain size on trigger stress for stress-induced martensitic transformation during tensile deformation in Ti-Al-Nb alloys was investigated. The trigger stress for stress-induced martensitic transformation (SIMT) in Ti-Al-Nb alloys exhibited a U-shaped behavior with variation in grain size. The variation of trigger stress with grain size was explained qualitatively, in terms of the contrasting change in the internal elastic energy stored in matrix due to formation of martensite (δEel) and the irreversible work done in overcoming the internal frictional resistance to phase boundary movement (∂Eirr) with β grain size. © The Minerals, Metals & Materials Society and ASM International 2008.