In this paper, the ionizing damage effects and the annealing behaviors of an import producti, p-type metal-oxide-semiconductor field-effect transistor (PMOSFET), an unhardened 4007 circuit under different doses are investigated. We measure the sub-threshold I-V characteristic curves of PMOSFET under different bias doses. The dependence of the drift of threshold voltage on total dose is discussed. We also observe the relationship between the parameter and the annealing time. The experiment results show that the PMOSFET of this kind can enhance low dose rate sensitivity (ELDRS) effect. The interface-trap formation by H+ transmission in the SiO2 is explained.We believe that the interface trap is a primary reason to induce ELDRS effect of PMOSFET. We also discuss the physical model of enhancing low dose rate sensitivity effect of PMOSFET.