In this study, direct observation of the evolution of electronic structures across complex oxide interfaceshas been revealed in the LaAlO₃/SrTiO₃ model system using cross-sectional scanning tunnelingmicroscopy and spectroscopy. The conduction and valence band structures across the LaAlO₃/SrTiO₃interface are spatially resolved at the atomic level by measuring the local density of states. This studydirectly maps out the electronic reconstructions and a built-in electric field in the polar LaAlO3 layer.Results also clearly reveal the band bending and the notched band structure in the SrTiO₃ adjacent to theinterface.