The variation of sheet temperatures, distribution of punch pressure and plastic strain during press forging of AZ31B sheet has been analyzed by computer simulation. The temperature of AZ31 sheet significantly increased during press forging. The estimated increment in temperature (AT) at central region of the sheet was in the range from similar to320 to similar to450degreesC depending on the test temperature. The lower the press forging temperature, the greater the temperature increment. During the press forging, foot regions of bosses showed greater temperature rise than other areas of AZ31 sheet, mainly due to localized severe plastic deformation. Center area of the sheet receives the largest contact pressure by the punch, while boss region receives relatively low pressure. As the press-forging temperature increases, the sheet receives lower contact pressure, indicating that the sheet was deformed more severely. The plastic strain of AZ31 sheet significantly increased as the forming temperature increased from 200 to 300degreesC, which is related with the change in microstructures, such as dynamic recrystallization occurring during the deformation process. The foot area of boss experiences greater plastic strains especially at the region between bosses. A design concept with optimum radius of curvature around boss should be employed to minimize cracking due to severe localized deformation during press forging.