Grain boundary diffusion of Al in polycrystalline Mg was measured from diffusion couple experiments between 603 and 693 K by high-resolution scanning electron microscopy microanalysis. Grain boundary diffusion of Al is found to increase with increasing misorientation between adjacent Mg grains, and the grain boundary diffusion coefficients of Al can be two orders of magnitude faster than the volume diffusion of Al in Mg. The difference between grain boundary and volume diffusion of Al becomes larger at lower temperatures.