SrZrO3 formation at the interface of gadolinia-doped ceria (GDC) interlayer and yttria-stabilized zirconia (YSZ) electrolyte is analyzed using high-resolution electron microscopy. SrZrO3 is dispersed in the inter-diffusion layer on the GDC side from the Ce/Zr border. Zr, which diffuses into the GDC grain, contributes to the formation of SrZrO3. The crystallographic relationship among the SrZrO3 grains and its neighboring GDC grains reveals that SrZrO3 is formed at the surface, at the grain boundary, and inside the grain, while maintaining a highly matched boundary with the adjacent GDC grain. The matching of the interface boundary is confirmed by the O-lattice theory, according to which the threshold Zr/Ce ratio is 13/34. If Zr/Ce ratio in the GDC grain is higher than the threshold, SrZrO3 may significantly grow into the grain. The conduction path for the oxygen ion is retained because the GDC grain containing Zr is split into the SrZrO3 grain and the less-Zr-containing GDC grain. If Zr/Ce ratio is lower than the threshold, SrZrO3 may be formed but will be limited by the amount of Zr diffusing from the adjacent region. Thus, the morphology of SrZrO3 is strongly affected by the state of GDC grains in the inter-diffusion layer. (C) The Author(s) 2018. Published by ECS.