The formability of a rare-earth alloyed magnesium sheet, ZEK100, is studied through determination of its Forming Limit Diagram (FLD). Limiting dome height experiments were performed at elevated temperatures with in situ digital image correlation (DIC) measurement of strain. FLDs were determined at three different temperatures (50°C, 150°C and 250°C) along the sheet rolling direction (RD) and transverse direction (TD). The experiments showed that temperature has a significant effect on formability; an increase in temperature from 50°C to 250°C resulted in a formability gain of more than 200%.