The superplastic deformation behavior of titanium alloy TC11 has been investigated in the temperature range of 810~1050 ℃ at the strain rate of 0.001 s-1 by tensile tests at high temperature, and the microstructures of samples deformed have been observed and analysed by metallographic microscope and transmission electron microscope. The results showed that the titanium alloy TC11 would not exhibit superplasicity in the b phase field, but exhibit in the temperature range of 810~980 ℃ in the a+b phase field with an optimal deformation temperature of near 900 ℃ and maximum elongation of 595%. The intracrystalline deformation, interface sliding, dynamic recrystallization or diffusion creep take part in the superplastic deformation at 900 ℃, and the interface sliding appears at a/b phase interface. The relative content of a and b phases has a heavy effect on the superplasicity and the titanium alloy TC11 exhibits an optimal superplasicity when the content of primary a phase is about 70%.