Abstract: Microstructure of EMP-TIG(tungsten inert-gas) welded Mg Al dissimilar metal joint was investigated by scanning electron microscope, electronic probe micro-analysis and X-ray diffraction. The results indicate that a Mg/Al joint with excellent microstructure performance can be achieved by gas tungsten arc welding and the microstructure in the weld metal is fine.There are three crystallized transition regions including columnar branch grain, equal-axis grain and columnar grain. The microhardness near the fusion zone changes little and a Mg/Al joint without brittle phase can be obtained by controlling process parameters.