The reactivity between lanthanum deficient lanthanum manganite, La0.9MnO3, and yttria doped cubic zirconia (YSZ) has been examined at the temperature range between 1250 degrees C and 1400 degrees C. The reaction product of La2Zr2O7 was observed after some induced period, which depended on the reaction temperature and the yttrium content in YSZ. The induced period increased with increasing Y2O3 content in ZrO2. The reaction mechanism has been discussed with reference to the the difference of the solubility limit of Mn and La in YSZ. The solid solution of Mn saturated YSZ showed no reaction product even when reacted with La0.9MnO3 at high temperatures for long periods. All the reaction products formed by the reaction showed lower oxide ion conductivity than that of YSZ and had the different thermal expansion coefficient values from that of YSZ.