In this work, transient electroluminescence (EL) (brightness-voltage waveform curve) was utilized to investigate the working mechanism of alternating-current biased organic light-emitting diodes (AC-OLEDs). In lower frequency domain, injection potential barrier was the dominant effect to determine the luminescence intensity; with increased frequency, the influence of capacitance effect becomes dominant, which can be confirmed according to the investigations on stable EL of the AC-OLEDs. The results indicate that transient and stable EL can agree with each other perfectly. Besides, the stable EL reveals that the thinner device can take more effective capacitance effect.