An ultrafine grained structure was obtained in the two grades of a 5083 Al alloy with or without scandium by using equal channel angular pressing and its superplastic behavior was characterized. For the alloy without scandium, low temperature superplasticity was obtained but high strain rate superplasticity was unlikely to occur. By contrast, the alloy containing a small amount of scandium exhibited high strain rate superplasticity. It was found that, in both cases, the strain rate region showing superplasticity was very limited, i.e. one order of magnitude. From the mechanical data, the deformation mechanisms were examined.