Pure yttrium aluminum garnet (YAG) nano-powders, doped with 1.4at.% of La2O3, ZrO2, MgO, Nb2O5, and SiO2, were vacuum sintered to full density and subjected to grain growth at 1700 degrees C, for up to 15h. The YAG intrinsic grain-boundary (GB) mobility, determined from the pure fully dense YAG specimens, was 2.9x10(-16) [m(3)N(-1)s(-1)]. All dopants, except for La2O3, increased the GB mobility compared to pure YAG (La2O3 didn't cause any significant change in YAG's GB mobility). All GBs were found to be free of secondary phases or intergranular films. These findings differ from numerous publications where dopants were found to either inhibit grain growth by solute drag, or to enhance grain growth due to liquid phase sintering. It was found that the GB mobility systematically increased with the decrease in the dopant cation radius. Moreover, it seems that vacancy population plays an important role in determining the GB mobility of YAG.