The reaction of La1‐xCaxMnO3(x= 0, 0.1, 0.2) with ZrO2‐8 mol% Y2O3(YSZ) has been investigated at temperatures ranging from 1300° to 1425°C in air. Substitution of Ca for La in LaMnO3depresses the reactivity with YSZ. A layer of La2Zr2O7is formed at the La1‐xCaxMnO3/YSZ interface after an induction period, and its formation is accelerated when the La1‐xCaxMnO3phase is porous. The reaction proceeds by unidirectional diffusion of La, Mn, and/or Ca ions, mainly Mn ions, into YSZ. The diffusion coefficients of La and Mn ions in YSZ, which are estimated using a LaMnO3/single‐crystal YSZ couple, are much lower than that of oxygen ion. From the experimental data, a reaction mechanism