For high density of resistive random access memory applications using NiOx films, understanding of the filament formation mechanism that occurred during the application of electric fields is required. We show the structural changes of polycrystalline NiOx (x = 1–1.5) film in the set (low resistance), reset (high resistance), and switching failed (irreversible low resistance) states investigated by simultaneous high-resolution transmission electron microscopy and electron energy-loss spectroscopy. We have found that the irreversible low resistance state facilitates further increases of Ni filament channels and Ni filament density that resulted from the grain structure changes in the NiOx film.