We have developed a labeling technique using stable isotope oxygen (¹⁸O) for "direct observation" of oxygen/oxide ion movements at the cathode/interlayer/electrolyte interfaces in a practical flatten-tube solid oxide fuel cells. The traces of oxygen motions were labeled during fuel cell reaction (current density of 0.25 A/cm² at 650 °C), and the ¹⁸O incorporation and diffusion were visualized in a "frozen state" by secondary ion mass spectrometry (SIMS). The active ¹⁸O ionization and incorporation sites were found to be in the CeO₂-interlayer between cathode and electrolyte. The higher ¹⁸O-concentration in the electrolyte was identified at the bottom of cell (higher current density at bottom).