Electric characteristics and resistive switching mechanism of Ni/HfO2/Pt cell are investigated. The cell has a forming-free property and shows an abnormal non-polar switching behavior. A high ON/OFF resistance ratio (>105) is obtained. The resistance of the on-state is independent of cell size, which implies that a conductive filament is formed in HfO2 film. X-ray photoelectron spectroscopy is used to investigate the compositions and valences of Ni and Hf in HfO2 film for the on-state cell. The results show that there is a hybrid filament comprised of a Ni filament and an oxygen vacancy filament in the HfO2 film for the on-state.