In this study, the degradation effects of inverted metamorphic tri-junction (IMM3J) solar cells were investigated after 1 MeV electron irradiation using spectral response and photoluminescence (PL) signal amplitude analysis, as well as electrical property measurements. The results show that, similar to traditional tri-junction (TJ) GaInP/GaAs/Ge solar cells, the electrical properties (such as I-sc, V-oc, and P-max) degrade as a function of log phi, where phi represents the electron fluence. In particular, the degradation of V-oc is more serious than that of I-sc because of the sum rule for V-oc and the limit rule for I-sc in IMM3J cells. Analysis of the spectral response curves indicates that, unlike traditional TJ cells, the bottom In0.3Ga0.7As (1.0 eV) sub-cell exhibits the most severe damage in the irradiated IMM3J cells. Meanwhile, the PL amplitude measurements qualitatively confirm that the degradation in the effective minority carrier lifetime tau(eff). of single junction (SJ) In0.3Ga0.7As cells is more drastic than that of SJ GaAs cells after irradiation. Thus, the output current of the In0.3Ga0.7As sub-cell should be the rate-limiting cell in the irradiated IMM3J cells. (C) 2016 Elsevier B.V. All rights reserved.