In this paper, we investigate the total ionizing dose (TID) effects of silicon-on-isolator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) with different sizes by using 60Co γ-ray. The SOI MOSFET contains a shallow trench isolation (STI) edge parasitic transistor and back gate parasitic transistor, in which STI oxide and buried oxide (BOX) are used as gate oxide, respectively. Although these parasitic effects are minimized by semiconductor device process, the radiation-induced trapped-charge can lead these parasitic effects to strengthen, thereby affecting the electrical characteristics of the main transistor. Since both the STI and BOX are sensitive to the TID effect, we try to distinguish their different influences on SOI devices in this work.The experimental results show that the characteristic degradation of device originates from the radiation-enhanced parasitic effect. The turning-on of the STI parasitic transistor leads the off-state leakage current to exponentially increase with total dose increasing until the off-state leakage reaches a saturation level. The threshold voltage shift observed in the narrow channel device results from the charge sharing in the STI, while the back gate coupling is a dominant contributor to the threshold voltage shift in short channel device. These results are explained by two simple models. The experimental data are consistent with the model calculation results. We can conclude that the smaller size device is more sensitive to TID effect in the same process.Furthermore, the influence of the negative bias at back gate and body on the radiation effect are also studied. The negative bias at back gate will partially neutralize the effect of positive trapped-charge in STI and that in BOX, thus suppressing the turning-on of STI parasitic transistor and the back gate coupling. The parasitic transistors share a common body region with the main transistor. So exerting body negative bias can increase the threshold voltage of the parasitic transistor, thereby restraining the TID effect. The experimental and simulation results show that the adjustment of the threshold voltage of parasitic transistor by body negative bias is limited due to the thin body region. The modulation of body negative bias in depletion region is more obvious in back gate parasitic transistor than in STI parasitic transistor. The weakening of parasitic conduction in the back channel is more noticeable than at STI sidewall under a body negative bias.