This study investigated the effects of extensional strain and strain rates on the foamability of PS-talc composite. Foaming processes were observed in situ for PS-talc composites with different surface treatment, size, and weight content of talc under extensional stress. Talc with the largest particle size produced the highest cell densities despite its lower talc particle density for heterogeneous nucleation. Meanwhile, the surface treatment seemed to slightly improve the talc's dispersion, but it did not cause significant differences in their foaming characteristics. The enhanced cell nucleation that took place with the larger talc was due to the higher tensile stresses generated around them in the presence of other growing bubble as compared to those around smaller talc particles. As the level of the applied extensional strain and/or the strain rate increased, the cell density increased significantly in all cases, and the cell density differences became smaller, especially when the talc content was high. The extensional stress caused the decrease of local pressure needed to induce nucleation around the smaller talc particles, so the effects of talc size became less apparent. (c) 2012 Elsevier Ltd. All rights reserved.