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The microstructure and mechanical properties of 5A06 aluminium alloy laser welded joint with thickness of 30 mm were investigated in this paper. The experimental results show that an amount of pore defects were found in weld seam zone, and the welded defects were mainly located at the grain boundary. The proportion of the welded defects with size less than 100 μm was beyond 50%. The microstructure in weld seam zone was mainly composed of dendritic grians, and several dendritic grians were located in a single grains. The misorientation between the dendritic grians was less than 5o, and the contrast between the dendritic grians was caused by element Mg microsegregation. The obvious interface was observed between the weld seam zone and heat-affected zone, and the contents of low angle boundary in heat-affected zone were higher than that of weld seam zone. Meanwhile, the grain morphology and size in heat-affected zone were different from the weld seam zone. Microhardness measurement shows that the definitive fluctuation of microhardness values in weld seam zone was observed, and the microhardness values of weld seam zone were beyond that of the microhardness values of matrx.