Equal channel angular pressing (ECAP) is one of the most promising processes to fabricate ultra-fine grained materials. The material deformation is affected by die geometry, material behavior, friction and back pressure. The optimum back pressure for 1100Al during ECAP was studied. The effect of back pressure on deformation behavior, effective strain and deformation load were analyzed by using finite element software. The results show that the corner gap between the billet and the die in the external part of the deformation zone decreases and even disappears with the increase of back pressure, which can produce more uniform and larger strain in the billet. The deformation load enhances with the increase of back pressure. From the simulation results, it can be found out that the optimum back pressure for 1100Al pressed in the die of Φ=90° is about 30MPa.