This article investigates the effects of nano-particles on cell morphology and foam expansion in the extrusion foaming of metallocene polyethylene/wood fiber nanocomposites with a chemical blowing agent. The results indicate that the addition of clay generally reduces the cell size, increases the cell density, and facilitates foam expansion. Furthermore, the foam material with added clay shows good char formation when it is burned. (c) 2007 Wiley Periodicals, Inc.