The superplastic deformation behavior of a quasi-single phase Zn-0.3 wt.% Al and a microduplex Zn-22 wt.% Al eutectoid alloy has been investigated in this study within the framework of the internal variable theory of structural superplasticity (SSP). Load relaxation and tensile tests were conducted at room temperature for quasi-single phase alloy and at 200°C for eutectoid alloy. The flow curves obtained from load relaxation tests on superplastic Zn-Al alloys were shown to consist of the contributions from interface sliding (IS) and the accommodating plastic deformation due to dislocation activities. The IS behavior could be described as a viscous flow characterized by the power index value of Mg = 0.5. In the case of quasi-single phase Zn-0.3 wt.% Al alloy with the average grain size of 1 µm, a large elongation of about 1400 % was obtained at room temperature. In a relatively large-grained (10 µm) single-phase alloy, however, grain boundary sliding (GBS) was not expected from the analysis based on the internal variable theory of SSP.