Abstract: The paper investigated the microstructure of the friction stir lap welded dissimilar joint between 6061 aluminum alloy and AZ31B magnesium alloy and the growth kinetic of intermetallic compounds (IMC) during post weld annealing treatment. The results show that the IMC layer consisted of continuous β-Al3Mg2 (near Al side) and γ-Al12Mg17 phase (near Mg side). The thickness of the IMC layer increased with increasing the annealing time and/or annealing temperature. In addition, the β layer was observed to grow faster than the γ layer. The increase of the IMC layer thickness was found to obey a parabolic relationship with annealing time, which reveals diffusioncontrolled mechanism during annealing. As the temperature increased from 300 ℃ to 400 ℃, the diffusion coefficient in the IMC layer increased from 2.88×1014 m2/s to 3.67×10-13 m2/s. The growth activation energy for the growth of IMC was 82.5 kJ/mol.