Structures and CO-adsorption reactivities of nickel oxide cluster cations were investigated by ion mobility mass spectrometry. The series of NinOn-2+, NinOn-1+ and NinOn+ cluster cations were predominantly observed in a mass spectrum at high ion-injection energy into an ion-drift cell. From the arrival time distributions of NinOn+ and NinOn-1+ in the ion mobility spectrometry, structural transition from two-dimensional (2D) ring to three-dimensional (3D) compact structures was found at n = 5. In addition, 2D and 3D structural isomers were found to coexist for Ni5O5+, Ni6O5+ and Ni7O6+. By adding CO gas to buffer gas in the ion-drift cell, Ni4O3+ and Ni5O4+ cluster cations were found to be more reactive for the CO adsorption reactions than Ni4O4+ and Ni5O5+. Under the pseudo-first-order approximation, rate constants for CO-adsorption were determined to be (8.4 ± 0.7) × 10-11 cm3 molecule-1 s-1 for Ni4O3+ and (9.6 ± 0.8) × 10-11 cm3 molecule-1 s-1 for Ni5O4+. These rate constants are 2 orders of magnitude faster than those for Ni4O4+ and Ni5O5+, which have reported previously. These differences of rate constants can be originated in the structures of the nickel oxide cluster ions.