Compared to fusion welding processes used in joining structural aluminum alloy, the friction stir welding (FSW) is an emerging solid state welding process which can lead to less distortion and residual stress owing to low heat input characteristic. In order to apply FSW in the aerospace field, the 2219 aluminum alloy in Al–Cu series, which has high–strength–weight ratio, resistance to stress corrosion cracking and superior cryogenic properties, and is an ideal material in the construction of liquid cryogenic rocket fuel tanks, has been welded in O condition (full annealing). Microstructures of weld nugget zones and mechanical properties of the slices in the three kinds of thick plate FSW joints, which are the flaw–free joint, the joint with flaw and the flaw–free joint after post weld heat treatment (PWHT), were studied. The tensile testing shows that the tensile strength σ_b and yield strength σ_0.2 decrease from 160.8 and 96.8 MPa of the top part of the flaw free joint to 146 and 86 MPa of the bottom part, respectively, σ_b and σ_0.2 decrease from 132.9 and 94 MPa of the top part of the joint with flaw to 126 and 78.8 MPa of the bottom part, respectively. σ_b of the middle of the flaw–free joint after PWHT is 243.8 MPa and σ_0.2 of the top of the flaw–free joint after PWHT is 123.3 MPa, they are higher than those of the other parts.  increases from 6.7%, 4.8% and 7.5% of the top parts of the flaw–free joint, joint with flaw and flaw–free joint after PWHT to 10.1%, 8.5% and 14% of the bottom parts of the joints, respectively. For the PWHT (500 ℃/50 min + 160℃/20 h) flaw–free joint, grains are more uniform and finer along the thickness direction in weld nugget zone, fractographs present deep dimples, most of the failure is ductile fracture, and the microhardness distribution has no obvious fluctuation. For the joint with flaw the mechanical properties decrease sharply, fractograph exhibits a ductile–brittle fracture morphology and the microhardnesses in slices are lower than those of the flaw–free joints.