Understanding of polypropylene (PP) foaming is critically important to reduce the weight of automotive parts. In this study, we used a batch foaming simulation system with visualization cell, to observe the foaming behaviors of PP that is blown with CO 2 and N 2 under various experimental conditions. We found that the nucleating agent content, initial temperature, pressure (i. e., gas content), and pressure drop rate during foaming have a significant effect on cell nucleation and cell growth. The cell density and the void fraction of PP foamed with CO 2 and N 2, respectively, were separately observed and compared. It was found that under the same experimental conditions, the maximum cell density of PP foamed with CO 2 was higher than that of PP foamed with N 2. However, the maximum cell density of PP foamed with CO 2 was determined to be lower than that of PP foamed with N 2, when the same gas mole numbers were employed. Based on the experimental results, optimum foaming conditions and effective processing strategies for PP-CO 2 system are suggested.