Correlation between galling which occurs during Mg alloy sheet forming using a steel die and friction coefficient measured during the sheet forming was investigated. Sliding friction (and/or wear) characteristics of the sliding couples including AZ31B Mg alloy and STD61 tool steel were evaluated using a disk-on-flat friction tester at 250℃. Both a Mg alloy sheet without any pretreatment and the alloy sheet with Mg(OH)₂ film on a surface were tested. Tribological behavior of the Mg alloy sheets slid against bare and DLC coated STD61 tool steel were examined in this study. Galling, which related with a material-transferring, was found to be controlled by mechanisms of adhesive junction forming, shear deformation and fracture of the junctions. The mechanisms also control the degree of resistance against relative sliding between the couples, which was measured as a friction coefficient. The correlation between the galling and the friction coefficient was discussed.