Rheocasting by applying limited angular oscillation was reported for AZ91 magnesium alloy, in which the molten metal was rapidly cooled and vigorously mixed during the initial stage of solidification. Microstructure and mechanical properties of treated AZ91 alloy were investigated. Experimental results indicate that the present rheocasting could give rise to significant microstructure refinement and spherical structure evolution. The hardness of the soft α-Mg phase was dependent upon the grain size and followed the Hall-Petch equation. The hardness of microstructure rheocast under oscillation intensity above the level of 0.26 rad/s2 can be predicted by a rule of mixtures (equal strain treatment). Simultaneous increase in both strength and ductility of AZ91 alloy has been obtained. The refinement of α-Mg phase combined with the discontinuity and refined β-Mg17Al12 phase is thought to be the main reasons for the improvement of mechanical properties. Possible mechanisms responsible for microstructure refinement were discussed.Highlights► Rheocasting using limited angular oscillation was reported for AZ91 alloy. ► The process led to significant microstructure refinement and spherical structure evolution. ► Simultaneous increase in both strength and ductility was obtained.