An AZ81 magnesium alloy, which was processed by hot extrusion in advance, was tested in the temperature range of 473 similar to 623 K and at various initial strain rates, ranging from I x 10(-1) to I x 10(-4) s(-1). True stress-true strain curves under various temperatures and activation energy within different temperature range were investigated for the analysis of effect of temperature on superplastic behaviors. Elongation of the as-extruded AZ81 alloy increased with temperature when tested at the same initial strain rate. Elongation values, obtained at the same initial strain rate of I X 10(-3) S(-1), were 94% at 473K and 446% at 623K, respectively. Deformation mechanisms varied with increasing temperature, from the grain boundary sliding (GBS) accommodated by grain boundary diffusion to the GBS accommodated by lattice diffusion, transition temperature was 573 K.