This research presents the foaming behaviors of linear polypropylene (PP) and PP/clay nanocomposites blown with supercritical CO2. The cell nucleation and expansion behaviors of the linear PP and PP-based nanocomposites at various clay contents during extrusion foaming are studied. The experimental results indicate that the nano-particles have a positive impact on improving the cell morphology, the cell density and the expansion ratio of the linear PP foams. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2972-2979, 2010