Abstract: The effect of processing times on the microstructure of friction stir processed AZ91D magnesium alloy was studied.The results indicate that the processing times has no prominent effect on the grain size of friction stir processed zone.However,more processing times could increase the area of the friction stir processed zone and homogenize its microstructure,change the microstructure of thermal mechanical affected zone to that of friction stir processed zone.Simultaneously,the process could also increase the shoulder zone and decrease its grain size.