It is generally difficult to predict the cell structure foamed from a binary polymer blend compared to a single polymer, as many factors in polymer blends add to the complexity of the foaming process without easy control, such as blend morphology, blend viscosity, and solubility and diffusivity of the foaming agent, etc. In this study, polypropylene (PP)/polystyrene (PS) blend foams with a bimodal cell structure were prepared by using supercritical carbon dioxide (scCO(2)) in continuous extrusion. By comparison of the cell structure of blend foams before and after extraction, it was found that the large cells were mainly formed in PS phase, while the small ones were mostly formed in PP phase. A possible mechanism was proposed to explain the formation of the bimodal cell structure. It was believed that a prominent difference in cell nucleation in the two phases of the PP/PS blends was required for the formation of bimodal cell structure. The proposed mechanism was confirmed by further experimental results, which showed that the bimodal cell structure could be changed by altering the nucleation ability in PP phase and PS phase through changing experimental conditions, such as the content of scCO(2) and the pressure drop, or through applying a heterogeneous nucleating agent. (C) 2015 Elsevier B.V. All rights reserved.