Negative permittivity is realized in unstructured imide polymer/carbon nanofiber (CNF) nanocomposites. The microstructure of CNFs is one of the factors that exert profound effects on the permittivity. Compared with cup-stacked CNFs-filled polyetherimide nanocomposites, the negative permittivity in the herringbone CNFs-filled nanocomposites is inhibited, occurring only at the frequencies near resonance, where the permittivity dips slightly into the negative region.