To study the effect of total dose effect and biasing effect on image sensor performance, Cobalt-60 gamma ray irradiation and annealing experiments were carried out on 0.18 μm process buried 4T- CMOS active pixel image sensor under different bias conditions. Emphasis was placed on the variation of parameters such as dark current and full well capacity with cumulative dose. The experimental results show that with the accumulation of total dose of irradiation, the dark current increases slowly in the early stage, and then the degradation is obviously aggravated. The main source of dark current in 4T -CMOS image sensor currents from STI interface and depletion width increase touching to STI, which aggravates degradation of dark current. Full well capacities have a drop after irradiation, which was due to the the photodiode capacitance decreases when the depletion width increases induced by radiation. And there is no remarkable biasing effect at 4T-CMOS image sensor. degradation of dark current.