Carbon nanofibers (CNFs)/polypyrrole (PPy) nano metacomposites are prepared by in situ polymerization in this article. The negative permittivity and negative permeability of CNFs/PPy nano metacomposites are achieved simultaneously. CNFs/PPy composites form different morphologies with the increase in CNFs content. Complex conductive networks formed by overlapped CNFs are present in CNFs/PPy composites with 30 and 50 wt % CNFs. The negative permittivity of the CNFs/PPy composites is attributed to the plasma oscillation of delocalized electrons, and the negative permeability results from a large number of conductive loops. The appearance of negative permeability is accompanied by the decrease in resistivity, which reflects that a large number of conductive loops are derived from complex conductive networks. Double negative property appears in the frequency range 845-1000 MHz in CNFs/PPy composites with 30 wt % CNFs content and 825-1000 MHz in CNFs/PPy composites with 50 wt % CNFs content. CNFs/PPy composites present double negative property in a wider frequency range compared with MWCNTs/PPy nanocomposites. (C) 2017 Wiley Periodicals, Inc.