Intercalated and exfoliated polystyrene/nano-clay composites were prepared by mechanical blending and in situ polymerization respectively. The composites were then foamed by using CO2 as the foaming agent in an extrusion foaming process. The resulting foam structure is compared with that of pure polystyrene and polystyrene/talc composite. At a screw rotation speed of 10 rpm and a die temperature of 200°C, the addition of a small amount (i.e., 5 wt%) of intercalated nano-clay greatly reduces cell size from 25.3 to 11.1 μm and increases cell density from 2.7 × 107 to 2.8 × 108 cells/cm3. Once exfoliated, the nanocomposite exhibits the highest cell density (1.5 × 109 cells/cm3) and smallest cell size (4.9 μm) at the same particle concentration. Compared with polystyrene foams, the nanocomposite foams exhibit higher tensile modulus, improved fire retardance, and better barrier property. Combining nanocomposites and the extrusion foaming process provides a new technique for the design and control of cell structure in microcellular foams.