Based on the adaptive re-meshing technique,the fully coupled thermo-mechanical model of friction stir welding of AA6061 is established. The wear loss of welding tool is calculated by Archard equation. The temperature curve and equivalent strain rate are exacted to calculate the grain size and distributions in recrystallization after weld by use of Monte Carlo method. Results indicate that the equivalent strain rate is increased with the increase of wear,which can lead to the increase of nucleation. The grain sizes in the stirring zone can be then decreased. The service life and weld quality can be increased by appropriate wear. Certainly,excessive wear can lead to the decrease of weld quality.