Time evolutions of electrochemical performance of La0.57Sr0.38Co0.2Fe0.8O3-delta (LSCF) during operation are evaluated based on three-dimensional microstructural analysis and electrochemical simulation. After 100 hours of operation at 700 and 800 degrees C, increases in polarization and ohmic resistances are observed. Segregated Sr phase is found inside the LSCF layer after the operation at 800 degrees C. LSCF, GDC, segregated Co phase, segregated Sr phase, GDC interlayer, interdiffusion phase, SZO phase, and YSZ electrolyte are segmented and reconstructed by focused ion beam - scanning electron microscopy (FIB-SEM) to analyze their microstructural characteristics in detail. No significant changes are observed for double phase boundary and triple phase boundary densities. Electrochemical simulations using the three-dimensional structures including all phases are carried out to predict the degradation factor. The increase in polarization resistance is well reproduced by assuming a decrease in the surface exchange rate, On the other hand, the increase in the ohmic resistance is attributed to the constriction of the ionic current in the inter-diffusion layer. (c) 2019 The Electrochemical Society.