In this work, the authors investigated the effect of Al doping on the resistance switching characteristics of sputtered NiOx films. Pt/NiOx/Pt metal-insulator-metal (MIM) stacks were fabricated by reactive dc magnetron sputtering with various Al contents in NiOx films and the resistance switching behavior of the MIM stack was characterized in conjunction with physical property, such as chemical bonding of NiOx. Al doping into NiOx films improved the endurance of SET/RESET operations, the distribution of VSET, and the memory window compared to the undoped NiOx films. X-ray photoemission spectroscopy showed that the Al doping caused the density of metallic nickel (Ni0) to be increased with the reduction of Ni3+ concentration. It was considered that the increase of metallic nickel (Ni0) in Al-doped NiOx films enhanced the repeated formation and rupture of conductive filaments during resistive switching operation, resulting in the enhanced endurance and the narrowed VSET distribution.