For simulating space environmental radiation a 2×1.7 MV tandem accelerator was used. GaAs/Ge solar cells were irradiated using 2 MeV C ions with the fluence from 3.1×109 to 6.9×1012 cm-2. Both I-V characteristics and spectral response were measured before and after irradiation. The results show that the Isc

Voc and Pmax of GaAs/Ge solar cells decrease as the irradiation fluence increases

and among them

Pmax decreases the most while Voc the least. This degradation rule is similar to that induced by proton irradiation. But to make Pmax of GaAs/Ge solar cells decrease to 50%

the required irradiation fluence of C ion is less than that of proton with the same range by two orders of magnitude. The spectral response of GaAs/Ge solar cells decreases more significantly in the long wavelength region than in the short wavelength region after C ion irradiation at a low fluence of 3.1×109 cm-2. When the fluence increases to more than 3.1×1010 cm-2

the spectral response in the whole wavelength region significantly degrades. As the fluence reaches to 2.3×1011 cm-2

the spectral response disappears.