Degradation modeling of InGaP/GaAs/Ge triple-junction (3J) solar cells subjected to proton irradiation is performed with the use of a one-dimensional optical device simulator, PC1D. By fitting the external quantum efficiencies of 3J solar cells degraded by 30 keV, 150 keV, 3 MeV, or 10 MeV protons, the short-circuit currents (I(SC)) and open-circuit voltages (V(OC)) are simulated. The damage coefficients of minority carrier diffusion length (K(L)) and the carrier removal rate of base carrier concentration (R(C)) of each sub-cell are also estimated. The values of I(SC) and V(OC) obtained from the calculations show good agreement with experimental values at an accuracy of 5%. These results confirm that the degradation modeling method developed in this study is effective for the lifetime prediction of 3J solar cells. (C) 2008 Elsevier B.V. All rights reserved.