© 2018 Trans Tech Publications, Switzerland This paper deals with characteristics of hot forging of twin roll cast magnesium alloys which have relatively high aluminum content. High tensile strength magnesium alloys containing 9 to 12% aluminum, such as Mg-9%Al-1%Zn, Mg-10%Al-1%Zn, Mg-11%Al-1%Zn, and Mg-12%Al-1%Zn have been made by twin roll casting. A new experiment was performed for hot forging of high strength magnesium alloys with high aluminum content was performed. From the results, using magnesium alloys with high aluminum content yielded less compressive deformation resistance than AXM403. It was also demonstrated that hot forging of magnesium alloys with high aluminum content produces small magnesium crystals (about six micro meters) and crystallized substances. The mean grain size of the microstructure of Mg-12%Al-1%Zn forged at 623K was less ten micrometers although that of the Mg-9%Al-1%Zn was about thirty micrometers. The small beta phase which precipitates in the twin roll cast Mg-12%Al-1%Zn was distributed uniformly comparing to Mg-9%Al-1%Zn. From the result of microscopic observation of the forged products, it has been recognised that the Hall-petch rule between mean grain size of forged materials and Vickers hardness has been proved. The effects of the dynamic recrystallization on the microstructures of the twin-roll cast products seem to be different in terms of aluminum content. Due to rapid cooling of twin-roll casting process process, the fabricated magnesium material could be used for hot forging. By applying a servo press machine, a hot-forging experiment was performed with development of high strength magnesium alloys. A novel material that show higher hardness have been fabricated by using twin-roll casting process. It has also been clarified that the aluminum content affect precipitation of beta phase as well as grain size.