The charge trapping effect of HfO2 gate stacks under heavy ion irradiation was evaluated via capacitance-voltage (C-V) measurement and transmission electron microscopy (TEM). Ion irradiation introduced oxygen vacancies into the HfO2 dielectric layer, which located primarily at the interface between HfO2 and Si. The oxygen vacancies act as trapping centers dominating the degradation of device performance after irradiation. The role of the traps on charge trapping in HfO2 gate stacks and the trapped charge stability (i.e. detrapping process) as a function of time were also investigated in this work. The mechanism of how MOS capacitors are affected by irradiation provides a good foundation for future space application of HfO2 gate stacks based integrated circuits.