PS-b-PMMA was grafted on Montmorillonite (MMT) by atom transfer radical polymerization (ATRP) to make exfoliated nanoclay composites. These nanocomposites were dispersed into polystyrene (PS) by solution blending. The mixtures were then batch formed with supercritical CO2. It was found that the cell density of foams based on these blends with 1 wt% modified MMT is higher than that based on neat PS and PS compounded with conventional organoclay at the same foaming conditions. As the nanoclay content increases, the foam density increases dramatically due to the high viscosity of the mixtures. The heterogeneous nucleation effect of the nanoclay and nanocomposites will be discussed.