We report on the mechanism of magnetization reversal in epitaxial Co/Fe bi-layers grown by molecular beam epitaxy on MgO(001) substrates. For Co films thicker than 5 nm, the crystal structure is hexagonal. The Fe layer follows an epitaxial relation relative to the MgO substrate of (001)  Fe// (001)  MgO. When deposited on a cubic Fe layer, the Co layer follows a bi-crystal epitaxial relation of (11 2- 0)  Co// (001) 〈 100 〉 Fe as previously reported [Popova, Appl. Phys. Lett. 81, 1035 (2002); Wang, J. Appl. Phys. 101, 09D103 (2007)]. The magnetization reversal in-plane follows a cubic fourfold symmetry, which coincides with that of the underlying bcc Fe layer. In this study, we find that the area of each Co crystal domain spans 200-1500 nm2 and that these two domains are approximately evenly distributed. The micromagnetic reversal mechanism is a combination of coherent rotational processes and domain wall displacement. These magnetic domains are sized tens of μm and separated by predominately 90° or occasionally 180° domain walls along the Fe 〈 110 〉 and Fe 〈 100 〉 directions, respectively. The cubic anisotropy of the bi-crystalline Co layer is explained by exchange-coupling between hcp grains with perpendicular crystallographic orientation, each having in-plane uniaxial magnetic anisotropy along its respective  direction.