Polymer nanofoams have recently attracted significant interest in both industry and academia. The unique nanoscaled porous structure could bring unprecedented material properties that have not been seen in conventional or microcellular polymer foams. It has been hypothesized that nanofoams could have a much higher specific strength and toughness as well as significantly improved thermal resistivity. In this research, we study the fabrication and characterization of polyetherimide nanofoams using a supercritical carbon dioxide foaming process. A process map indicating the conditions to obtain various polyetherimide foam structures, including micro-, micro/nano transition, and nanofoams has been established. Two types of nanofoams were observed, one made with high gas concentrations and the other with high foaming temperatures. The one with high gas concentrations exhibited a higher specific modulus than that of unfoamed polyetherimide. Nanofoams generally showed a higher thermal resistivity than microfoams with similar relative densities. It is found that the equilibrium CO2 concentration in polyetherimide under the supercritical conditions does not fit well to the well-known dual-mode sorption model. A new gas concentration model was developed to describe the CO2 uptake under supercritical conditions.