The deformation mechanism in the nanometer grain size range has been basically investigated from the results of microstructural observation after superplastic deformation in a Zn-Al eutectoid alloy in which a reverse grain size dependence of superplasticity was previously reported: flow stress increases and elongation decreases with decreasing grain size when grain size is markedly reduced to nanometer range. By controlling the aging condition after solution treatment and subsequent quenching, two specimens are prepared: the as-quenched specimen with ultrafine grains of 83nm in diameter and aged specimen with normally fine grains of 2.6μm. The elongation is confirmed to be smaller in the as-quenched specimen than in the aged specimen, although the flow stress is lower. As a result of TEM observation on the interior of the grains, dislocations are rare in the as-quenched specimen, while a significant density of dislocations are observed in the aged specimen. This result strongly supports the mechanism previously proposed by Mishra et al. that the accommodation process, i.e., the dislocation glide inside the grains, becomes more difficult with decreasing grain size in the nanometer grain size range, even though the grain boundary sliding as the major process becomes facilitated. Roughly assessed m-value was in accord with this mechanism.