Microstructures and oxygen diffusion were investigated at the LaMnO3 film/Y2O3-stabilized ZrO2 (YSZ) interface by transmission electron microscopy (TEM), atomic force microscopy (AFM) and secondary ion mass spectrometry (SIMS) analysis for samples after heat treatments and isotope oxygen exchange (O-16/O-18 exchange) experiments. In "as-deposited LaMnO3 film" on YSZ, the O-18-diffusion profile showed a significant decrease in O-18 concentration near the LaMnO3 film/ YSZ interface, which corresponds to an amorphous layer. The heat treatment diminished the amorphous layer at the LaMnO3 film/YSZ interface. In the heat-treated sample (1473 K for 5 h), the O-18-diffusion profile in the LaMnO3 film showed gradual decrease and a subsequent minimum of the O-18 concentration at the LaMnO3/YSZ interface. This is due to the grain growth of lanthanum manganite, which provides the shorter paths to YSZ.