Size effects on the compressive strength of nanostructured Mg-micropillars were investigated. Mg–10Al alloy micropillars with diameters ranging from 1.5 to 8 μm were prepared by focused-ion-beam and tested under micro-compression. A significant improvement in strength was found by reducing the pillar diameter3.5 μm. The deformation mechanisms of the compressed pillars were characterized using transmission electron microscopy. We attribute the size-induced strengthening to a less number of dislocation sources along with a higher activity of non-basal deformation mechanisms.