In La0.6Sr0.4Co0.2Fe0.8O3.delta (LSCF) cathode/Gd-doped ceria (GDC)/yttria-stabilized zirconia (YSZ)-electrolyte based solid oxide fuel cells (SOFCs), one of the key issues affecting performance and long-term stability is the apparent deactivation of LSCF cathode by the presence of secondary phases such as SrZrO3 at the interfaces. Herein, we report that by modifying the cathode-interlayer interface with a dense LSCF thin film, the severe cation interdiffusion is suppressed especially the fast gas or surface diffusion of Sr into adjacent GDC-interlayer/YSZ-electrolyte resulting in the significant reduction of SrZrO3 formation at the interfaces improving cell stability. In order to understand the present results, the interface chemistry is carefully considered and discussed. The results show that modification of cathode-interlayer interfaces is an important strategy for improving the lifetime of SOFCs.