A Mg-Al intermetallic compounds coating was prepared on the surface of Mg-steel lap joint by arc-sprayed Al-Mg composite coating (Mg-cathode and Al-anode) and its post-heat treatment (PHT). The effect of PHT temperature on the phase transition, microstructure and mechanical properties of the coating was investigated by X-ray diffraction, scanning electron microscope, energy dispersive X-ray spectroscopy, optical microscope and microhardness test. The result shows that the intermetallic compounds layer that is mainly composed of Al3Mg2 and Mg17Al12 is formed by the self-diffusion reaction of Mg and Al splats in the coating after PHT for 4 h at 430 degrees C. (C) 2010 Elsevier B.V. All rights reserved.