Ruddlesden-Popper (RP) faults in perovskites as one type of the anti-phase boundaries provide a versatile platform to manipulate the magnetic, conducting, and magnetoresistance properties in the complex oxides. Local fine structure at the boundaries containing light atoms is a key factor to understand the underlying structure-function relationship due to their interfaces mediated effects. In this work, the atomic structure, chemical distribution and electronic structure of typical RP faults in perovskite LaNiO3 and its epitaxial nanocomposite films on a SrTiO3 substrate were systematically investigated combining high-resolution scanning transmission electron microscopy (STEM) and density functional theory (DFT) calculations. Integrated differential phase contrast (iDPC) STEM imaging results demonstrate a NiO2-x-(LaO-LaO)-NiO2-x atomic configuration with content fluctuations of oxygen at the RP faults. Atomic electronic energy loss spectroscopy results and DFT calculations illustrate that Ni cations have mixed valence of Ni2+, Ni3+ at the RP faults accompanying with oxygen environment fluctuations affected by the non-stoichiometric bonding. This study offers a comprehensive route to explore intriguing chemical structures and physical properties in the homointerface structure at the atomic level.