Structural assignments of gas-phase magnesium oxide cluster cations, MgnOn + (n ≤ 24), have been achieved from a comparison of experimental collision cross sections (CCSs) measured using ion mobility mass spectrometry and theoretical CCSs calculated for equilibrium structures optimized by quantum chemical calculations. Various structures based on rock-salt and hexagonal-tube structures were assigned for n = 5-13 and 15. On the other hand, only rock-salt type structures were assigned for n = 4, 14, 16-21, and 24. The CCS values and the total energies were close for the hexagonal-tube and rock-salt structures of a given size in the small size range (n ≤ 15 except for 4 and 14). The hexagonal-tube structures of the cluster ions with n ≥ 16 were less stable and had larger CCSs than the rock-salt structures. These results indicate that the structures of the MgnOn + clusters were changed from mixtures of rock-salt and hexagonal-tube structures to pure rock-salt structures with the growth of the cluster size. All atoms in the hexagonal-tube structures are located on the surface of the clusters, even if the cluster size increases. In contrast, the assigned rock-salt structures with n = 13, 14, and n ≥ 17 had atoms inside the clusters, which means that the average coordination numbers are substantially higher for the rock-salt structures than for the hexagonal tube structures. The structural change from the mixed structures to pure rock-salt with an increase in size n can be attributed to this difference in the coordination number.