In this study, the use of carbon nanofibers (CNFs) as nucleating agents to produce polystyrene nanocomposite foams was demonstrated. With the addition of CNFs, microcellular foams with uniform cell size distributions were obtained. Compared to nanoclay and single-walled carbon nanotubes (SWCNTs), CNFs exhibit substantially higher nucleation efficiency in the foaming process. The underlying mechanism is semi-quantitatively explained by the classical nucleation theory. The homogeneous fiber distribution and favorable surface and geometrical characteristics of CNFs make them ideal nucleating agents. (c) 2005 Elsevier Ltd. All rights reserved.