A new prediction model for estimating luminance degradation of an OLED subjected to varied combinations of forward and reverse biases is proposed. To this end, the known increase in the voltage of OLEDs under long-time operation versus degraded emitted luminance is first-time ever considered to identify the degradation exponent as logarithm functions of elapsed time. With the identified degradation versus time, the degraded OLED luminance can be well predicted even subjected to varied combinations of forward and reverse biases. The established model is particularly useful for achieving required lifetime of modern AMOLED displays. Based on the model in terms of equations, the OLED parameters including forward, reverse bias stresses, and initial luminance can be applied to the model to accurately estimate the luminance variation during long-time operation. The estimation error between model predictions and experimental measurements is less than 1.6 %.