We showed previously that a rigid and fibrous-structured cationic coiled-coil artificial protein had cell-penetrating activity that was significantly greater when compared with a less-structured cell penetrating peptide. Nanomaterials with anisotropic structures often show aspect-ratio-dependent unique physicochemical properties, as well as cell-penetrating activities. In this report, we have designed and demonstrated the cell-penetrating activity of a shorter cationic coiled coil protein. An aspect ratio at 4.5:1 was found to be critical for ensuring that the cationic coiled-coil protein showed strong cell-penetrating activity. At an aspect ratio of 3.5:1, the cationic coiled-coil protein showed cell-penetrating activity that was similar to a less-structured short cationic cell-penetrating peptide. Interestingly, at an aspect ratio of 4:1, the cationic coiled-coil protein exhibited intermediate cell-penetrating activity. These findings should aid in the principle design of intracellular drug delivery carriers including coiled-coil artificial proteins, their derivatives, and alpha-helical cell-penetrating peptides as well as provide a framework for developing synthetic nanomaterials, such as metal nanorods and synthetic polymers.