Real-time measurement of γ irradiation effect of Hg1-xCdx Te long-and mid-wavelength focal plane array photodiodes has been carried out. Through measuring the current-voltage characteristic during irradiation process, it has been found that mid-wavelength detectors are more radiation resistant than long-wavelength photodiodes. For long-wavelength detectors, the zero bias resistance, which is usually used to evaluate the performance of photodiodes, decreases with increased γ irradiation dosage. For mid-wavelength detectors, the zero biased resistance does not show a definite changing trend, and the irradiation mainly caused fluctuations of resistance-voltage curves with increased dosage. By numerically simulating the resistance-voltage curves of long-wavelength detectors on the basis of dark current mechanism, it was found that the lifetime ofminority carriers in the generation-recombination process was shortened and thedefects produced by irradiation increased as the dosage increased, and the affected dark current mechanism was mainly the generation-recombination current. Because the irradiated mid-wavelength detectors have much larger carrier mobility and much lower dopant density, and also a bandgap twice that of long-wavelength detectors, they showed a weaker irradiation effect. The fluctuations of the resistance-voltage curves caused by irradiation would lead to an increase on noise of the detectors.