Microcellular polyetherimide (PEI) foams were prepared by microcellular injection molding using supercritical nitrogen (SC-N-2) as foaming agent. The effects of four different processing parameters including shot size, injection speed, SC-N-2 content, and mold temperature on cell morphology and material properties were studied. Meanwhile, multiwalled carbon nanotube (MWCNT), nano-montmorillonoid (NMMT), and talcum powder (Talc) were introduced into PEI matrix as heterogeneous nucleation agents in order to further improve the cell morphology and mechanical properties of microcellular PEI foams. The results showed that the processing parameters had great influence on cell morphology. The lowest cell size can reach to 18.2 m by optimizing the parameters of microcellular injection molding. Moreover, MWCNT can remarkably improve the cell morphology of microcellular PEI foams. It was worth mentioning that when the MWCNT content was 1 wt %, the microcellular PEI/MWCNT foams displayed optimum mechanical properties and the cell size decreased by 28.3% compared with microcellular PEI foams prepared by the same processing parameters. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4171-4181, 2013