Microstructure of weld nugget zone and mechanical properties of the whole and slice in the thick plate joints obtained by friction stir welding (FSW) were studied. The results show that the tensile strength σb, yield strength σ0.2 and elongation δ decrease with increasing weld speed when the rotary speed is constant. The maximum σb, σ0.2 and δ lie in the top weld and reach 186.7MPa, 100.3MPa and 14.1% respectively, while the minimum σb, σ0.2 and δ in the bottom joint only attain 157.5MPa, 80.2MPa and 10.1% separately. The fracture morphologies show that there are lots of meshy dimple and the deepest locates in the top joint. The secondary intercrystalline crack and quasi-cleavage face lie in the root of weld. The microhardness at the top is higher than that at the bottom and presents asymmetrically through the weld centerline. The fine and equiaxial grain size is bigger at the top than that at the bottom and clings together at the root of weld. Compared with the bottom, second-phase particle at the top redissolves into the matrix abundantly and the strengthening effect boosts up.