Microstructure of AZ91 magnesium alloy was modified by using semi-solid rotating container process (RCP) and addition of 1 wt.% Ca. Thermal analysis (CA-CCTA) was performed during semi-solid process in order to determine the relationship between microstructural modification and RCP parameters. The container containing molten AZ91 alloy was rotated by rotation speed of 0, 150, 180 and 210 r/min at 640-580 °C. The cooling curves were plotted to determine the solidification characteristics, solid fraction and dendrite coherency point (DCP). The results indicated that the cooling rate increased with increasing the rotation speed significantly and led to an increase in maximum solidification range of 15.9 and 6.4 °C for AZ91 and AZ91-1Ca (wt.%), respectively. Applying shear force to the slurry at 180 r/min minimized the solid fraction and delayed appearance of DCP. It affected the microstructure characteristics such as grain size and circularity factor. The rotation of 180 r/min at 585 °C resulted in the globular microstructure with a circularity factor of 0.72 and a grain size of 73.4 μm. The addition of 1 wt.% Ca to AZ91 in the optimum condition of RCP increased the circularity factor to 0.81 and reduced the grain size to 53.2 μm.