Hemming is used either to improve appearance (to create a smooth edge rather than a razor edge with burrs) or to attach one sheet metal part to another. It is often used as the last stage of stamping operations. In an automotive industry, hemming is used to assemble one sheet metal part to another to make sub-assemblies such as doors, hoods and deck-lids. Hemming operations are done in three stages such as flanging, pre-hemming and hemming. The bent or flanged sheet is folded to 180 degree or more and the bent sheet profiles are three dimensions on hemming. Therefore, the deformation mechanism is different and more complicated than simple bending processes. Very little information is available in research literatures, and most of the hemming dies are designed based on trial and error. The study examines the mechanism and characteristics such as deformation distributions, profiles accuracy, spring back and forming forces of hemming by numerical simulation. The material of aluminum sheet metal is A6063 and the thickness is 1.0mm. The flanging, pre-hemming and hemming is simulated by the three-dimensional elastic-plastic finite element method. The stress and strain distributions, profiles and forming loads are examined at hemmed sheet metals. The mechanism and characteristics of hemming are made clear in this paper.