The deformation and damage mechanism of aluminum alloy (6063) were investigated by 0°, 30°, 45°, 60° and 90° tensile tests and tensile-unload tests with the modified Arcan fixture on the butterfly specimens. The results show: the curves of engineering stress-engineering strain under different stress states are obviously different. There were microvoids in the specimen when 0°direction loading was preformed. The microcracks were produced in the root of notch as the result of the microvoids shearing fracture and then they led to specimen fracture with microcracks being coalesced. With tensile angle increasing, the shear stress in the center of butterfly specimen increases gradually, while the deformation bands become more and more concentrative. In these concentrative deformation bands, the microcracks are produced and then microcracks propagation and coalescence result in specimen fracture. When 90° direction loading is preformed, the shear bands are obviously formed. The G-T-N damage model and the Johnson-cook model were used to simulate 0° tensile test and 90° tensile test respectively. The simulated engineering stress-engineering strain curves fit the measured ones very well.