The formability of AZ31 magnesium alloy sheets, with two different thicknesses, has been investigated at room temperature and 250°C by means of Nakazima tests. The different straining conditions have been studied by using sheet blanks with several length to width ratios, and Forming Limit Diagrams were then obtained with and without using lubricant. As expected, an increase in temperature was observed to enhance the formality of the alloy. The formability increases also by increasing the thickness as well as by using Teflon foil as lubricant. The microstructure of the deformed samples was analysed by means of light optical microscope.