Abstract: The "abrupt stop and rapid cooling" technique was used to study the microstructure evolution during friction stir welding (FSW) of 2024 aluminum alloy. Electron backscattered diffraction (EBSD) maps were taken in different regions behind and ahead of the tool keyhole,and the characteristics and mechanism of microstructure evolution were analyzed. The results show that discontinuous recrystallization occurred in early stage of FSW. The continuous recrystallization,resulted from the gradual increase of subgrain boundary misorientation due to absorption of dislocation,was the dominating mechanism of dynamic nucleation during FSW. Grains grew to the final structures during the subsequent cooling cycle,in which partial static recrystallization occurred.