Abstract: The study of the total ionizing dose effect of 0.18 μm MOS differential pair transistors which are exposed to a ⁶⁰Co γ-ray radiation was presented in this paper, in the view of the characterization of mismatch and degradation mechanism. The results show that NMOS differential pair transistors are more sensitive than PMOS differential pair transistors. In NMOS differential pair transistors, the mismatch on the transfer characteristic curve and threshold voltage is degraded after irradiation, whereas this phenomenon does not take place in PMOS differential pair transistors. In addition, the gate current has the large degree of radiation hardness associated to very thin gate oxide.