AZ31 alloy sheet fabricated by rolling was processed by friction stir processing (FSP) with different passes. The effect of FSP on the microstructure and damping capacity of AZ31 alloy sheet was discussed. The fine and equiaxed grains were obtained in the stirred zone (SZ) for FSPed samples from 1 pass to 3 passes with the average grain size of 10.6, 10.4 and 13.6μm, respectively. The damping peak P1was presented on the curves of temperature-dependent damping capacity for FSPed samples. The damping peak P2 was restrained after FSP and the damping peak P1 was a relaxation process. The FSPed samples (2-pass FSP and 3-pass FSP) obtained high damping capacity. The best damping values10Q-(ε=10-4) and damping values1HQ-(ε=10-3) of the sample subjected to 3-pass FSP (0.0131 and 0.0496) increased by 33.7% and 157.0%, respectively.