Two methods to predict the pressure drop threshold (Delta P-threshold) to initiate bubble nucleation in polymer foaming processes are developed. One method uses the modified nucleation theory developed in our previous work, while the other utilizes computer simulations to model the growth profiles of the first observable bubbles in batch foaming experiments. These two approaches have shown reasonably good agreement qualitatively with each other in their Delta P-threshold predictions. Moreover, the effects of the pressure drop rate, the gas content, and the processing temperature on Delta P-threshold are demonstrated. It was found that the pressure drop rate does not have a significant effect on Delta P-threshold, while increasing the gas content or the processing temperature leads to a decrease in Delta P-threshold.