The superplastic deformation behavior of 'FinAl based (~+/3) alloy was studied with respect to the volume fraction of ez2/fl. Three alloys containing 21, 50 and 77% in volume fractions of/3 exhibited large tensile elongations of over 500% at 970"C with a strain rate of 2.5 • 10 4 sec 1, The largest elongation was observed in the alloy with 21% of /3. As the volume fraction of ]3 phase increased, the flow stress and correspondingly, the strain-rate sensitivity values decreased. Due to the higher diffusivity of Ti in ,/3 phase than in ~h phase, the increase in ]3 volume fraction from 21% to 77% accelerated the dynamic grain growth, and degraded the superplasticity of the Ti3Al-based alloys. The strain-based grain growth behavior was quantitatively analyzed and incorporated into a constitutive equation. The calculated flow curves are in agreement with the experimental ones in the stable deformation region.