The generally short lifetimes of organic light-emitting diodes (OLEDs) presents a challenge to their widespread acceptance for use in large-area displays and solid-state lighting. A greater understanding of the degradation mechanisms would help to further improve the reliability of OLEDs particularly at high brightness levels by optimizing the material selection and structural design, and pave the way for their broader applications as lighting sources. In this work, we studied the stability of green phosphorescent OLEDs with different structures under constant-current (20-50 mA/cm2) stressing. Through the modifications of the ITO anode by different plasma treatments and the hole transport layer (HTL) by incorporating inorganic component or dopants, we proved that energy level misalignment at the ITO/HTL interface leads to localized joule heating, accelerating defect generation and luminescence decay. Pulsed current stressing was then employed to suppress the joule-heating effect so as to differentiate the thermal and nonthermal factors governing the device degradation. The luminance evolution comprised an initial rapid decay regime and a subsequent slow decay regime, and only the latter was governed predominantly by electrical excitation. In OLEDs with an appropriate energy level alignment at the ITO/HTL interface, pulsed stressing with 10% duty cycle only improved the effective half life by ~15% as compared to continuous-wave stressing, indicating a minor role played by joule heating.