Replacement of noble metal catalysts for oxygen reduction reaction (ORR) with transition metals can have a profound impact on clean and economical energy production. Although non-precious metals have been extensively studied as ORR electrocatalysts, there has been a long debate about what active site is responsible for the ORR and what the catalytic mechanism is. The mystery of the chemical nature of the active sites hinders the development of efficient and stable ORR catalyst. The biggest obstacle to resolving the mechanism is the fact that the active sites are buried in the bulk, porous catalyst and the components in the real catalyst are complicated. Here, we propose a thin film model approach to study the active sites of FeN thin film, deposited on top of glassy carbon by magnetron sputtering. Rutherford backscattering, X-ray diffraction and Raman spectroscopy were carried out to characterize bulk films and to determine deposition conditions that result in Fe tetrahedrally coordinated with nitrogen. The significance of this study is that this is the first time active sites are intentionally exposed to the surface and can be directly characterized by x-ray photoelectron spectroscopy (XPS). Bonding between Fe-N in the bulk film and N-C at the interface between FeN and glassy carbon have been confirmed by XPS. Rotating disk electrode experiments have been carried out to measure the ORR activity of Fe-N, C-N, Fe-C the Fe-N-C exposed surfaces by looking at thin Fe-N, C-N, Fe-C, and Fe-N-C films. This study will also provide answers to the controversy about whether carbon, nitrogen or iron is essential in the ORR catalyst.