Laser cladding of aluminium powders was performed on pure magnesium substrates using the blown powder method. The microstructure across the laser-clad track was studied. Starting from the bottom of the laser-clad to the top surface of the track, a series of microstructures was observed: a planar growth interface of the substrate, a narrow band of (β+γ) eutectic, a narrow band of β cellular structure, and finally a relatively thick layer of α-Al and (α+β) eutectic structure. The evolution of the various phases and microstructures is discussed in terms of the distribution of solute in conjunction with the Al-Mg phase diagram, temperature gradient and solidification rate, constitutional supercooling theory, and the condition for columnar-to-equiaxed crystal transition.