Structures of cobalt oxide cluster cations have been investigated by ion mobility mass spectrometry. When the ions were injected into a drift cell with 250eV kinetic energy, (CoO)n+ and ConOn-1+ were observed. Collision cross sections were experimentally determined for (CoO)2-7+ and ConOn-1+ (n=5–7). Orientation-averaged collision cross sections were calculated for optimized structures of these ions obtained in quantum chemical calculations. By comparison between experimental and theoretical cross sections, (CoO)3-5+ ions are found to have monocyclic-ring structures. By contrast, (CoO)6,7+ have compact tower structures. Therefore, structural transition from the ring to compact structures occurs at (CoO)6+.