Degradation modes due to dark defects under accelerated aging for InGaAsP/InP double heterostructure lasers are investigated by monitoring pulse threshold current, leak current, absorption coefficient, gain factor, and electroluminescence topograph. Most of the dark defects are dark spot defects (DSD’s) and there are only few ⟨100⟩ dark line defects. At the initial stage of the degradation, these dark defects scarcely absorb the emitted light, and the reduction of gain factor causes the increase of pulse threshold current. After this stage, dark defects begin to act as absorber of the emitted light. The generation time of such DSD’s strongly depends on the injected current density but only weakly on the junction temperature in the range of 25 ° to 250 °C. The activation energies for the generation time of the first dark spot defect and the growing speed of ⟨100⟩ dark line defects are estimated to be 0.16 and 0.2 eV, respectively.