Magnesium alloy is currently expected to be widely used for high lightweight and high efficient materials in the automobile and electronics industry. Although Magnesium is excellent materials, it can’t be formed in the room temperature due to HCP structure. However in order to improve the magnesium formability, it is required by the investigation of forming at warm temperature range. This study aims to confirm characteristic of formability of AZ31 magnesium alloy sheet by the warm tensile test and microstructure analysis. The formability of AZ31 magnesium was evaluated at various temperatures, strain rate, strain rate sensitivity and thickness. Furthermore the result of microstructure of AZ31 magnesium shows how grain growth is changed by thickness.