We propose a theoretical approach that overshoot phenomena can be theoretically predicted for an electro phosphorescent OLEDs. The transient EL responses of phosphorescent OLEDs adopting the mixed-host emissive layer were studied in order to investigate the carrier recombination mechanism. It was observed that a prominent overshoot peak in the transient EL profiles, depending on the composition ratio of the HTM and the ETM, when the applied bias voltage decreased abruptly to zero. A good agreement is found between the theoretical and experimental results.