The study on effect of gamma(γ) ray irradiation on silicon photodiodes for on-orbit calibration was carried out. Silicon photodiodes were irradiated by 20 krad(Si), 35 krad(Si) and 50 krad(Si) gamma doses resepctively. The darkness current and spectral responsivity were measured before and after irradiation. It′s found that the darkness current and spectral responsivity have no dramatic change under less than 35 krad(Si) gamma doses. Under 50 krad(Si) gamma doses, the darkness current of the sample increases slightly, but the influence on the application of calibrator can be ignored. The results suggest that silicon photodiode under test can be used as a candidate device for on-orbit calibrator in visible spectral bands due to its good long-term stability and reliability in the space irradiation environment.