Polypropylene (PP) nanocomposites containing 2.5 and 5.0 wt.% multiwalled carbon nanotubes (MWCNT) were foamed using supercritical carbon dioxide (CO2) in a single-step batch process. SEM and TEM were used to investigate the CNT dispersion and cellular morphology. Through-plane electrical conductivity and mechanical performance of nonfoamed and foamed nanocomposites were measured. Electrically conductive nanocomposite foams with expansion ratios of up to ten times and cell sizes between 200 nm and 50 μm were successfully fabricated. Two different types of foams were identified: a) nanocellular foams with low expansion ratios (1.4-2.2) and b) microcellular foam with high expansion ratios (3-10). Overall, the electrical conductivity of the foams presented a density dependent behavior, with an optimum degree of expansion resulting in a maximized electrical conductivity. In the nanocellular nanocomposites with low expansion range, the electrical conductivity increased by about one order of magnitude when foaming was introduced. In the high expansion range, however, the electrical conductivity proportionally decreased with the foam density. Also, the specific tensile properties of the foams, especially toughness were significantly improved by foaming in the low expansion range. The evolution of the orientation state of CNTs in the foamed nanocomposites was analytically quantified based on the degree of biaxial stretching exerted by the cell growth. The analytical results were then used to discuss the inter-connection of CNTs and to explain the interrelationship between the electrical conductivity and degree of foaming.