Abstract: The tensile deformation behavior of TC11 titanium alloy has been investigated in the strain rate range of 0.000 1-0.4 s-1 at 900 ℃,and microstructure of the deformed alloy was observed and analyzed by optical microscope and transmission electron microscope.The results showed that the alloy exhibited superplasticity as the strain rate was not larger than 0.04 s-1,and the superplasticity increased with the decrease of strain rate.When the strain rate was 0.000 1 s-1,the elongation could reach as higher as 1 215%.The α phase in the transversal section of the alloy was equiaxed,but the α phase in the longitudinal section turned from strip shape to equiaxed shape with the decrease of strain rate.During the superplastic deformation process,there were dynamic recrystallization and diffusional creep.The boundary slip occured at the α/β phases boundaries,and both the α and β phases occured deformation.The deformation mode was the mixed type of isostress and isostrain rate modes.