The origin of the resistance switching behavior in HfO2 is explained in terms of filament formation/rupture under an applied voltage. In order to investigate the position and process of conductive filament in resistive switching memory, the resistive switching and chemical structure of Ti/HfO2/Pt memory device are studied. Through current-voltage measurement, typical resistive switching behavior is observed in Ti/HfO2/Pt device cells; through detecting Hf 4f with different depths by using X-ray photoelectron spectroscopy. It is observed that the Hf4+ decreases monotonically with depth increasing towards HfO2/Pt interface in low resistance state, while a fluctuation distribution of Hf4+ is shown in high resistance state and in the pristine Ti/HfO2/Pt device. The concentration of Hf4+ in high resistance state is higher than that in low resistance state, which is confirmed by measuring the electron energy loss spectrum. Additionally, the O 1s spectrum shows a similar result consistent with the Hf 4f one. The above result is explained by the existence of locally accumulated oxygen vacancies in the oxide bulk layer in high resistance state and pristine states. It is proposed that the oxygen vacancy clusters dominantly determine the resistivity by the connecting/rupture between the neighbor cluster sites in the bulk. The cluster defects are the preexisting structural distortion/injure by charge trapping defects due to the fixed charge which could confine the nucleation of oxygen vacancies and bigger distortion could be enhanced or recovered via the transportation of oxygen vacancies under the external voltage. Oxygen vacancies are driven away from the clusters under SET electrical stimulus, and then recover back to original cluster sites under RESET process.#br#The previous presumption of the ideal evenly-distributed state for oxygen vacancies in the bulk of resistance random access memories (RRAMs) device leads to an issue about where the filaments occur/form first since the oxygen vacancy defects show uniform distribution in the active oxide bulk layer. Since the conductive filament is easily formed in the cluster region of oxygen vacancies, this study could provide a deep understanding of the formation of conductive filament in RRAMs device.