The gamma ray Total Ionizing Dose (TID) effect up to 10 Mrad on 0.18-um MOSFET is modeled and a radiation-hard design method for stabilizing oscillation frequency of CMOS inverter base ring oscillators have been devised. The TID effect on MOSFET is modeled using a function of the two exponential sums to express combination phenomena of two kinds of charge trapping for the threshold voltage and leakages current variations, respectively. By using this model, the TID effect to CMOS inverter circuit was analyzed, and it was clarified that the oscillation frequency of the CMOS ring IC can be stabilized against the TID effect by optimizing the size ratio of p-type and n-type MOSFETs. This was successfully confirmed by fabricating and evaluating CMOS oscillator ICs with the 0.18-um technology. These results are useful design guideline for CMOS ICs for systems in radiation-irradiated environments.