The strip casting of AZ31 were carried out by a Ø250×150mm of vertical twin roll caster at different casting speeds (9-15m/min) and in different casting temperatures (630-660°C). The solidification microstructure of the strips was examined. The experiment results showed that the crystal grain size of the casting was smaller than that of conventional ingot, and decreases while the casting speed is raised, or the casting temperature decrease. The casting temperature strongly affected the dendrite structure that changed into sphere-like when the casting temperature was lower. The appropriate casting temperature for AZ31 magnesium alloy is 640°C, nearby its liquidus temperature.