The high temperature deformation of Al-3% Mg and Al-6% Mg alloys has been investigated by differential tensile tests. The activation energies, activation volumes and frequency factors at different temperatures have been obtained. The controlling deformation mechanism in the temperature range 250-400℃ is proved to be possibly due to the non-conservative motion of jogged screw dislocations. The deformation equation is ε=Nιjb2zAνexp{-(△Hs-ιjb2τ)/(kT)}. Thereafter the jog spacings, dislocation densities, jog densities and dislocation velocities at different temperatures are calculated for both alloys. In the above equation ε is the strain rate, b the Burgers vector, Z the coordination number, A the entropy factor, v an atomic frequency, △Hs the self-diffusion coefficient, τ the applied shear stress, k Boltzmann's constant and T the absolute temperature.