The microstructural changes associated with phase transformation and the straining behavior in polycrystalline structures during transformational superplastic deformation have betransformatonal superplasticen investigated. In-situ observations have been carried out with specially designed hot-stage microscopes. The strain distribution has been examined by microscopic strain analyses using a micro-grid pattern with 12.7 and 6 μm intervals in pure iron, 0.1%C steel and eutectoid steel. The results are as follows: (1) In the initial stage of Ac3 transformation in pure iron, superplastic strain is induced by the sliding at o/y interface along the prior ferrite grain boundaries and by the grain rotation. In the intermediate stage, the sliding deformation is generated at the migrating tranformation interface associated with the growth of austenite grains. (2) Accumulated strain by sliding has been observed within the previously transformed region during Ac3 transformation in 0.1%C steel. (3) The characteristic straining behavior in the eutectoid steel is due to the behavior at the transformation interface in the microstructure including secondary phase Fe3C particles. These observations suggest that a sliding mechanism at the migrating interface is a principal mechanism of transformation superplasticity.