High temperature tensile ductilities and deformation mechanisms of an extruded and rolled AZ31 Mg alloy were investigated. Elongation-to-failure tests were conducted under constant T-head velocity and constant temperatures ranging from 300 ℃ to 450 ℃. Strain-rate-change tests were conducted under varying strain rate from 5×10-5 s-1 to 2×10-2 s-1 and constant temperature from 300 ℃ to 450 ℃. Experimental results show that the maximum elongation of the AZ31 alloy with an average grain size of about 19 μm is 117% at strain rate of 10-3 s-1 and temperature of 450 ℃. Stress exponent and activation energy were characterized to clarify the deformation mechanisms. The enhanced ductility is dominated by solute drag dislocation creep, and the major failure mechanism is cavity growth and interlinkage.