Impact of total ionizing dose (TID) on the within-wafer variability of radiation-hardened (RH) silicon-on-insulator (SOI) wafers is investigated in this article. The within-wafer variability is measured by the front gate and back gate I-V characteristics of the transistors in 32 dies evenly distributed on wafer locations. The experimental results show the complex dependence of the within-wafer variability on TID: the within-wafer variability of RH SOI wafer is first weakened by TID irradiation and then rebounds. The evolution of net trapped charge induced by TID in buried oxide (BOX) is affected by positively charged silicon nanoclusters introduced by silicon ion implantation, which is responsible for the above complex dependence. Moreover, radiation hardness assurance method relying on sample testing of limited wafer locations is discussed, which can give the reasonable estimation of the within-wafer variability on TID irradiated devices.