Present investigation has been made to study the superplastic deformation behavior of Ti-6Al-4V alloy based on the theory of inelastic deformation, and to analyze the boundary sliding characteristics by the use of transmission electron microscopy. The flow curves obtained by load relaxation tests at 600℃ and 800℃ are fitted well with the inelastic deformation theory. It has been found that the deformation mode for the fine grained microstructures (3㎛) is a type of iso-stress mode- while that of coarse grained mi­crostructures (11 ㎛) is a mixed type of both iso-stress and iso-strain rate modes. The density of dislo­cations observed near the grain boundaries is higher along α/β phase boundaries than the α/β grain boun­daries, which confirms that the sliding resistance of α/β is lower than that of α/β.