The phase structure and dislocations in rapidly solidified AZ91D magnesium alloy were studied by TEM and XRD techniques. It shows that under rapid solidification conditions, the common eutectic transformation La-Mg+ Mg17Al12 in AZ91D alloys is suppressed to a great extent. As a consequence, the rapid solidification microstructures consist dominantly of supersaturated a-Mg solid solution, whose size is in the range of 0.8~6.0 m. TEM analysis reveals that little amount of oval Mg17Al12 granules, trace polygonal Al8Mn5 particles and very small anomalistic -AlMg phase with the size of 5~70nm precipitate in a-Mg grains or their boundaries. The -AlMg phase is metastable transition configurations which is further transformed into stable Mg17Al12 phase. There are large numbers of dislocation pile-up and cells with the size of 0.2~0.7m in a-Mg grains, and the high dense dislocations mainly concentrate on the (002) close-packed face and exhibit a remarkable orientation.