We report herein the structural influence of in-plane cation arrangements on ion conductivities of layered double hydroxides (LDHs). Mg-Al-CO3 and Mg-Ga-CO3 LDHs with different proportions of trivalent cations were synthesized by the co-precipitation method, and we found that their ion conductivities presented a steep increase only at peculiar proportions of trivalent cations. Electron-beam diffraction patterns revealed that LDHs with such proportions had an ordered honeycomb arrangement of di- and trivalent cations in (0001) hydroxide layers. We propose a crucial insight that relates the order/disorder arrangement and ion conductivities for better understanding the nature of LDHs.