La₂Mo₂O₉-based system is one of the widely investigated oxide ion conductors for its excellent ionic conductivity at intermediate temperatures. The detailed phase relationships and transition phenomena also attract interests in combination with oxide ion conduction. In recent years, we found that low-temperature stable phase (β’-phase) with the quite similar structure with β-phase appears by annealing the supercooled β below 460 ^oC for the sample x ~ 0.06 of La_2-xBi_xMo₂O₉, and the transformation process from β into β’ can be expressed based on Avrami’s equation. In addition, Ba- or Pb-substituted systems also exhibit the low-temperature β’ phase through the annealing of supercooled β-phase. The electron diffraction and thermal hysteresis of conductivity suggested that subtle ordering of oxide ion is related to the formation of β’-phase. In this article, we show our recent research on the low-temperature phase transition phenomena on La₂Mo₂O₉-based oxide ion conductors.