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.　©　2023　Elsevier　B.V.