This thesis presents relationships between ionic conductivity and crystal structure of hexagonal perovskite revealed from a new perspective: order and disorder. It describes three state-of-art research results in the new area of structure of ionic conductors, hexagonal perovskite-related structures, based on a new viewpoint of structural order/disorder. The first is occupational disorder of oxygen atoms and ionic conductivity in Ba3MNbO8.5 (M = Mo, W) investigated by high-temperature neutron diffraction and the maximum-entropy method. Second is the hidden chemical order in disordered Ba7Nb4MoO20·0.15H2O that was discovered by a combined technique of resonant X-ray diffraction and solid-state nuclear magnetic resonance. This combined technique was named as RXRD/NMR method. The third is the discovery of a rubidium-containing oxide-ion conductor with large positional disorder of oxygen atoms. This volume gives readers two outlooks. One is proposed guidelines for the search and development of ionic conductors with hexagonal perovskite-related structures. Another is future application of analyses for order/disorder used in this study, especially the RXRD/NMR method.