In order to obtain fine-microstructure magnesium alloys with superior mechanical properties, AZ61 alloy was processed by friction stir processing (FSP) combined with rapid heat sink. It is found that ultrafine-grained microstructure with average size less than 300nm is observed in the resultant AZ61 alloy. The mean microhardness of the ultra-fine region reaches Hv l20-130. two times higher than that of AZ61 substrate. All these results demonstrate clearly that under a cooling rate high enough, ultra-fine structure in AZ61 alloy with superior mechanical properties call be produced by one pass FSP via dynamic recrystallization.