SSR (Semi-Solid Rheocasting) is a relatively new simple and low cost semisolid process for producing semisolid slurries and ingots which rely on formation of a high nuclei density at the early stages of solidification. Due to lack of research on this process, especially for magnesium alloys, in this study the feasibility of SSR processing of AZ91 magnesium alloy was investigated. The experimental procedure can be divided into two stages. In the first stage, AZ91 alloy was subjected to three different melt treatment procedures before being cast into a steel die at three different temperatures, i.e. a temperature above the liquidus temperature and two temperatures within the semisolid temperature range of the alloy. The main treatments included normal cooling to the casting temperature, immersing a stationary cold copper rod into the melt during cooling of the alloy (ICR) and inserting a rotating copper rod during cooling of the melt (SSR). The microstructures of the cast specimens were then characterized. In the second stage, the effect of applied pressure during secondary solidification of the semisolid slurries on the microstructure and mechanical properties of the specimens were studied and compared with those of the conventionally squeeze cast specimens. The result showed that non-dendritic microstructures could be obtained with both ICR and SSR processes but SSR process generated smaller and more spherical primary particles. In each method, sphericity and grain size of primary particles increased with decreasing the casting temperature (increasing the solid fraction). Formation of non-dendritic primary particles in the microstructures decreased the hardness of the alloys. In the SSR process, increasing the applied pressure had a little effect on the morphology and grain size of the primary particles, but it had a considerable effect on the intermetallic phases and eutectic structure. The results showed that strength of conventionally squeeze cast specimens increased with increasing the applied pressure but similar trend was not observed for the SSR specimens. Study of the fracture surfaces suggested this to be associated with the higher possibility of formation and entrapment of oxide bifilms in the SSR processed slurries. The effect of applied pressure on the hardness of conventionally squeeze cast specimens was more than on SSR specimens. Density of SSR specimens cast under 30MPa pressure was more than that of conventionally squeeze cast specimens at this pressure. Considering the results, it was recognized that SSR processing of flux protected magnesium melts was not a suitable method due to the high possibility of formation of oxide layers and entrapment of inclusions and flux in the melt which would result in loss of mechanical properties. For utilization of full benefits of SSR process, the entire process should be performed under a suitable