Abstract: TIG welding has been carried out for thin plate of aluminium alloy 2A12, and the correlation between microstructure and mechanical properties was analyzed. Coarse and mixed grains were observed in welding area by means of metallographical analysis. From the edge to the center of welding seam, the volume grains changed to isometric grains, a mixed grain distribution was shown near the fusion line. Inhomogeneous mechanical behavior was studied by micro hardness and tensile testing. The results showed that minimal hardness was located in the fusion line, and fracture was also happened in the same area, which was accordance to the microstructure distribution. SEM and electron probe tests were done for fracture analysis and elements identification of welded joints, it was shown that the main reason for strength decreasing of aluminium alloy welded joint was the precipitation phases θ and s, which were generated during cooling process of welding.