The electrical and optical properties of electron or proton-irradiated Cu2ZnSnS4 (CZTS) solar cells were investigated. The normalized efficiency of CZTS solar cells decreased when fluence was greater than similar to 10 (15)cm(-2) for electron irradiation or 10(12)cm(-2) for proton irradiation. This tendency is quantitatively the same as that of Cu(In,Ga)Se-2 (CIGS) solar cells, and better than that of Si solar cells. One of the origins of degradation is the formation of deep defects such as Cu atoms substituted at Zn sites and nonradiative recombination centers in the CZTS layer, as suggested by the decreasing photoluminescence peak intensity at 1.2 eV with increasing irradiation fluence. On the other hand, the solar cell performance improved in the case of a small amount of electron or proton irradiation due to the bombardment soaking effect. These results indicate that the CZTS solar cells show excellent tolerance of electron and proton radiation, similar to a CIGS solar cell.