Polymer nanocomposite foams are promising low density substitutes for nanocomposites. Carbon nanotube/polymer nanocomposite foams possess high strength, low density, and can be made conductive. Good control of foam properties is of great importance in the application of such materials. In the current study, multi-walled carbon nanotubes (MWNTs) with controlled aspect ratio were used to alter the foam morphology in MWNT/poly(methyl methacrylate) (PMMA) nanocomposite foams produced by a supercritical carbon dioxide (CO2) foaming process. It was found that with the addition of one weight percent of MWNTs, the Young's modulus of polymer foams increased by as much as 82%, and the collapse strength increased by as much as 104%. The influence of MWNT aspect ratio on the compressive properties of nanocomposite foams was investigated. The addition of MWNTs influenced the foam properties in two ways: improving the compressive properties of the solid matrix, and reducing the bubble size of the nanocomposite foams. A modified constitutive model for predicting the compressive properties of high density closed-cell polymer foams was developed. The influence of the bubble size on the mechanical properties of polymer foams was discussed based on the new model. (c) 2011 Elsevier Ltd. All rights reserved.