[en] Polyetherimide (PEI) and carbon nanofiber (CNF) composites have been developed successfully by using a Sigma high temperature internal mixer and then compression molded. The amount of carbon nanofibers used was 1-3 phr (parts per hundred of polymer), respectively. Thermal properties were characterized by using thermogravimetric analysis (TGA). Thermal conductivity was measured at temperatures between 50 and 180 ^oC. Thermal conductivity increased with the incorporation of CNFs. Scanning electron microscopy (SEM) showed the state of dispersion of CNFs, in the entire volume of matrix. Dynamic mechanical analysis (DMA) demonstrates that both the storage modulus (E') and glass transition temperature (T_g) of the PEI/CNF composites is increased. The storage modulus of the polymer is significantly increased by the incorporation of acid treated CNFs particularly at high temperatures, indicating there is some chemical bonding between PEI and CNFCOOH (acid treated CNFs). The study showed that acid treatment of carbon nanofibers enhanced the dispersion and interfacial bonding between fibers and the matrix, and hence improved the electrical conductivity properties. Modification results in a significant decrease in resistivity compared to as received CNFs composites. The electrical conductivity of the composites was measured as a function of temperature