The grain boundary internal friction of pure Al and the same Al doped with different amounts of Ga ( 10, 50 and 410 ppm) was measured before and after high temperature deformation at two different modes ( static tensile creep and cyclic reverse torsion). It was found that without deformation, the smaller amount of Ga ( 10 and 50 ppm) slightly increases, but the larger amount of Ga ( 410 ppm) obviously increases the activation energy. After tensile creep, the peak height for Al-10 ppm Ga and Al-50 ppm Ga decreases to a lower level, but keeps at a higher level after cyclic torsion, than that for pure Al and Al-410 ppm Ga. The results are interpreted in terms of perturbations in the grain boundaries due to slip and grain boundary roughening for stationary boundaries and an enhanced sliding by grain boundary migration. Migration and roughening are promoted by Ga contents up to 50 ppm while the effects are diminished by higher amounts of Ga. This is consistent with previous observations that smaller amounts of Ga increase the grain boundary mobility in pure Al. (C) 2003 Kluwer Academic Publishers.