Higher conversion efficiency has been predicted theoretically for InGaP/In0.16Ga0.84As/Ge lattice-mismatched triple junction solar cells owing to widening of the effective range of the solar spectrum than the conventional lattice-matched solar cells. The most serious problem in lattice-mismatched system is the large lattice mismatch (∼1%) between In0.16Ga0.84As and Ge cells. The effect of a thermal cycle annealing (TCA) process which is expected to reduce the defect density in this system has been discussed from electrical and structural viewpoints. The minority carrier lifetime in In0.16Ga0.84As emitter layers were improved after TCA treatment from TR-PL measurement. EBIC measurements showed a reduction of the structural defect such as misfit dislocations due to the TCA process in In0.16Ga0.84As base layers. The misfit components observed in the base layers may have some influence through the emitter layers.