We have investigated the effects of ionization on the aggregate structures of tetradecyldimethylamine oxide (C14DAO) at the mica-solution and graphite-solution interfaces by atomic force microscopy. It was found that the C14DAO forms aggregate structures with lower curvature at the mica-solution interface as the degree of ionization of C14DAO increased. Nonionic C14DAO formed short cylindrical aggregates on mica. On the other hand, a flat bilayer on mica was observed for the half-ionized C14DAO1/2HCl and the fully ionized C14DAOHCl. The flat bilayers for C14DAO1/2 HCl and C14DAOHCl suggest the higher packing parameter (or lower mean curvature) character of these surfactants, compared to the cationic quaternary ammonium surfactant with a single tetradecyl tail as well as the nonionic C14DAO which all form the cylindrical aggregates on mica. The reason for this lower curvature of half-ionized C14DAO was suggested to be the hydrogen bonds between the headgroups (i.e., the cationic-nonionic pair for C14DAO1/2 HCl), in addition to the contribution of the electrostatic attractive interaction between the cationic headgroup and the negatively charged mica. Contrary to the aggregates on mica, the dominant structures of C14DAO an graphite were hemicylinders, irrespective of the ionization. It seas found that the ionization increased the distances between the hemicylindrical aggregates due to the electrostatic repulsion between the aggregates rather than induced the change in the aggregate structure. The weak dependence of the ionization on the aggregate structures at the graphite-solution interface is suggested to be due to the dominant attractive interaction between the graphite surface and the surfactant tail.