The crossbar structure of resistive random access memory (RRAM) is the most promising technology for the development of ultra-high-density devices for future nonvolatile memory. However, only a few studies have focused on the switching phenomenon of crossbar RRAM in detail. The main purpose of this study is to understand the formation and disruption of the conductive filament occurring at the crossbar center by real-time transmission electron microscope (TEM) observation. Core-shell Ni/NiO nanowires were utilized to form a cross structure, which restricted the position of the conductive filament to the cross-center. A significant morphological change could be observed near the crossbar center, which resulted from the out-diffusion and backfill of oxygen ions. Energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) demonstrated that the movement of the oxygen ions led to the evolution of the conductive filament, followed by redox reactions. Moreover, the distinct reliability of the crossbar device was measured via ex situ experiments. In this work, the switching mechanism of the crossbar core-shell nanowire structure is beneficial to overcome the problem of nanoscale minimization. The experimental method shows high potential to fabricate high-density RRAM devices, which could be applied to 3D stacked package technology and neuromorphic computing systems.