The deformation mechanism and the role of grain boundary sliding (GBS) and intragranular deformation characteristic of low temperature superplasticity (LTSP) were investigated in ultrafine-grained (UFG) Al-Mg alloy using a multi-axial alternative forging technique. In UFG materials, it was shown that elongation and strain rate sensitivity were 340 % and 0.39 respectively at 473 K under a strain rate of 2.8 x 10(-3) s(-1). On the other hand, when grain size exceeded 3 mum, superplasticity did not appear under the same conditions. The main factors affecting the deformation mechanism were investigated based on observations of the microstructure using SEM and TEM. The intragranular deformation contribution, estimated from the aspect ratio of the grains after deformation, was observed to be about 43 %. The appearance of LTSP indicates that the role of intragranular deformation and grain size, together with GBS, were the most important factor.