Crystal orientations after cold rolling to a 50% thickness reduction were measured at mid-thickness parallel to the rolling plane for two aluminum single crystals: one having a {100} ⟨001⟩ orientation and the other having an orientation deviated 5° about the rolling direction axis from ideal {100} ⟨001⟩. The crystal rotation axis orientations calculated from electron backscatter diffraction were compared with ⟨112⟩ lattice rotation axis orientations geometrically assigned to individual slip systems. The crystal rotation axis orientations were explained by the resultant lattice rotation axis orientation consisting of the four active slip systems having high Schmid factors. The microstructure in the single crystal having {100} ⟨001⟩ was subdivided by primary active slip systems, whereas the microstructure in the single crystal with deviated orientation was subdivided by secondary active slip systems, which developed band structure parallel to the rolling direction. The crystal rotation axis orientation method is useful for determination of the type and slip amplitude of active slip systems.