Computer simulation calculation shows that if there were no rigid translation of adjacent grains, the shortest distance between lattice points at all the incoherent twin boundaries would be 0.408a, where a is the lattice parameter, except for coherent ones. This distance is smaller than atomic diameter 0.707a, therefore atomic overlap occurs. The interface energy can be greatly reduced by a rigid, translation of these two crystals. The displacement vectors of four interfaces (111), (122), (121) and (765) were determined experimentally, they are 0.254a, 0.300a, 0.204a and 0.126a respectively.