The alloy Ti-6-Al-4V deforms superplastically in the temperature range 750 to 950° The most important factor which is responsible for superplastic behavior was found to be the very fine grain size. Strain rate has no direct effect on superplasticity, however when the strain rate is very low (approximately 2 × 10 s), prolonged exposure to high temperature causes grain growth and early failure. The strain rate sensitivity factorm = 0.5 and the apparent activation energyAH = 45,000 cal/mole, which is approximately the same as the activation energy for grain boundary self diffusion of titanium. Both values are independent of strain rate within the range 10 - 2.5 × 10 s. All the experimental points fall in a straight line when plotted as log (ekTd* 2/DgbGb3) vs log (σ/G) with a slopen = l/m = 2. This is in excellent agreement with the theory of grain boundary sliding accommodated by dislocation motion.