Cyclo‐olefin copolymer (COC) composites containing different amounts of nickel (Ni) and/or nickel‐coated carbon (NiC) hybrid fillers were prepared, and their morphological, rheological, mechanical, dielectric, and X‐band microwave absorption properties were examined. Rheological percolation thresholds for Ni and NiC fillers were determined to be in the range of 45 to 60 and 60 to 75 phr, respectively. Both fillers caused no significant increase in melt viscosity at low shear rates. The minimum filler concentrations to obtain acceptable reflection loss (RL) values (RL < −10 dB) were determined as 60 and 45 phr for the Ni and NiC series of samples, respectively. The minimum RL value (RL_min) and the broadest absorption bandwidth were determined as −52.5 dB and 8.2 to 12.5 GHz, respectively, for the composite sample containing 75 phr of Ni, whereas a similar performance could be obtained by the sample containing 45 phr of NiC. Considerable enhancement in the RL_min value could not be provided by the use of hybrid NiC fillers; however, the absorption bandwidth was enlarged, and the minimum sample thicknesses to obtain an acceptable absorption bandwidth were reduced significantly.