Radiation resistance of high-efficiency InGaP/GaAs tandem solar cells with a world-record efficiency of 26.9% (AM0, 28 degrees C) has been evaluated by 1 MeV electron irradiation. Degradation in tandem cell performance has been confirmed to be mainly attributed to large degradation in the GaAs bottom cell. Similar radiation resistance with GaAs-on-Ge cells has been observed for the InGaP/GaAs tandem cell. Moreover, recovery of the tandem cell performance has been found due to minority-carrier injection under light illumination or forward bias, which causes defect annealing in InGaP top cells. The optimal design of the InGaP base layer thickness for current matching at end of life (EOL) (after irradiation with 10(15) electrons cm(-2)) has been examined.