For the first time the retention of high resistance state (HRS) in resistive random access memory (ReRAM) is found to compose of three stages - extending tail-bits, distribution shift, and then distribution broadening. This work provides a comprehensive study on the HRS's retention behavior in WOx-based ReRAMs from single device characteristics to group distribution. Different from conventional activation energy (Ea) analysis, the mean and variance of the array distribution are presented to overcome the non-uniform Ea issue. Since the extracted Ea fits well with Vo
(oxygen vacancy) migration characteristics, the three retention stages are suggested to be the competing results from the migration, recombination, and generation of the Vo
in the gap region. A three-dimensional retention model with kinetic Monte Carlo simulator and trap-assisted tunneling conduction is proposed to discuss the dominating mechanism in different time and temperature scales. The mechanism for random telegraph noise is also included to illustrate the fluctuating nature of the HRS cells.