Abstract: Uniaxial tensile tests and microstructure observation were carried out in the temperature range of 860-950 ℃ and strain rate range of 5×10-4-5×10-3 s-1 to evaluate the superplasticity of TC4 titanium alloy with different initial grain sizes (2,8,18 μm).The influence of grain size on superplastic behavior and deformation mechanism of TC4 titanium alloy was investigated.The results indicated that under the optimum deformation conditions of strain rate 5×10-4 s-1 at 890 ℃,the maximum elongation for the fine-grained (2 μm) TC4 alloy was 1 300%,and 450% for the coarse-grained (18 μm) alloy.After superplastic deformation,for the fine-grained (2-8 μm) TC4 alloy,the average strain rate sensitivity index m was about 0.50,microstructure was equiaxed and some dislocations were observed near the α/β boundaries.For the coarse-grained (18 μm) TC4 alloys,m was only 0.30,the microstructure had little equiaxed grains,a large number of dislocations were observed not only near the α/α boundaries but also in α grains,and subgrains were also observed in α grains.