MOSFET's and MOS memory devices suffer the increase of interface state density and oxide trapped charge density after ionization radiation such as gamma rays. The drain‐current dependence and the gate‐voltage dependence of the flicker noise of the irradiated MOSFET's are illustrated in this paper. The annealing effect, the bias effect, and the total dose effect on the noise behavior are shown. It is also shown that the change of the interface state density is essential to the noise change and the change of oxide trapped charge density may cause the “apparent” change of MOSFET's flicker noise after gamma irradiation.