Semi-solid injection molding (Thixomolding) is a molding method for magnesium alloys. Our team has been investigating the forging of thixomolded magnesium body in order to improve its mechanical properties so that the thixomolded magnesium parts expand the application in transportation industry. Thixomolded magnesium alloy can include coarse alpha phase grain when the alloy is molded below liquidus temperature. Although it is expected that the mechanical properties of a forged thixomolded magnesium alloy are affected by the existence of the coarse alpha grains, its microstructure evolution during forging process has not been clarified yet. We studied the dynamic recrystallization (DRX) behavior of a thixmolded magnesium alloy using a cylinder compression test and microstructure observation. The microstructure observation showed that DRX occurred at the grain boundary and within the alpha phase. The Electron backscattering diffraction (EBSD) data showed that DRX is caused not only by diffusion but also by twinning in the alpha phase.