The structural prerequisites have been well established for fine-structure superplasticity of metallic materials. Hot extrusion and hot rolling are primary processes as conventional refinement methods of metallic materials. In the present work, rotary-die equal-channel angular pressing (RD-ECAP) was used for refining the microstructure of AZ91 alloy, which is very effective established by experiments. The microstructure of AZ91 contains alpha-phase and Mg17Al12-phase existing at alpha-phase grain boundaries. Average grain size of about 2-4 mum was attained after RD-ECAP for 16 passes at 573K. Superplasticity of the fine-grained AZ91 alloy was investigated. The dimension of specimen is 14 mm in gage length L and 2.5 mm. in diameter D (L/D = 5.6). Tensile tests for superplasticity were carried out in air at temperatures between 543 to 603K, and at strain rate between 5.6 X 10(-4) to 3.4 X 10(-2)s(-1). The maximum total elongation of 370% was obtained at the strain rate 1.20 X 10(-3)s(-1) at 573K. The strain rate sensitivity (m-value) was different at each test temperature. At 573 and 603K, the strain rate sensitivity was 0.36 and 0.31 at the strain rate range from 5.67 X 10(-4) to 5.67 X 10(-3) s(-1), respectively. During superplastic deformation, the grain size of alpha-phase increased from about 2-4 mum to about 11 mum. The average size of Mg17Al12-phase did not increase much, but the distribution was improved.