Polyetherimide (PEI)/organoclay (Cloisite 30B) nanocomposite and PEI/pristine nanoclay (K10) nanocomposite were made by solution casting process. Nanoclay content of 0.5, 1.0, 2.0, and 3.0% (by weight) was reinforced in PEI. The dielectric properties (dielectric constant, loss tangent, and electromagnetic transmission/reflection losses) of PEI/Cloisite 30B were observed lower than PEI/K10 in X band frequency (8 to 12 GHz). Mixed morphology in terms of intercalation and exfoliation was evident in wide-angle X-ray diffraction (WAXD) analysis and transmission electron micrographs (TEM) of PEI/Cloisite 30B. Whereas, in PEI/K10, immiscibility, segregation, and phase separation of K10 were observed in WAXD analysis and scanning electron micrographs (SEM). Tensile properties (tensile strength, modulus, and elongation at break) of PEI/Cloisite 30B were superior to PEI/K10. Reinforcement with 1%, by weight, of Cloisite 30B in PEI resulted in the optimum properties, among all the compositions. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 1040-1046, 2011