The gamma-ray irradiation causes positive charge traps formation in silicon dioxide films and at silicon dioxide - silicon interface of MOS devices, and the threshold voltage shift in MOS transistors. Here, the Monte Carlo model was used to develop an approach for estimating gammaray induced traps spatially distributed in silicon dioxide films. This is combined with the model of energy distributed traps at silicon dioxide - silicon interface. The developed model enables gammaray induced charge and threshold voltage shift determination as a function of gamma-ray doses. The threshold voltage measurements at a single specified current, both of radiation sensitive and radiation hardened MOS transistors irradiated with different doses of gamma-ray are compared with the developed model and good agreement are obtained.