In this study, we report a feasible way to synthesize carbon nanotube nanocomposites deposited with cobalt nanoparticles (20-30 nm) on the surface (Co/CNTs) to serve as an electromagnetic (EM) wave absorption material. EM absorption measurements indicated that epoxy resin composites with 20 wt% Co/CNTs exhibited an effective EM absorption (RL < -10 dB) in the frequency range of 2.5-20 GHz with an absorber thickness of 1.0 to 6.0 mm. A strong absorption peak (RL = -36.5 dB) appeared at 4.1 GHz as the thickness was 4.0 mm, and the absorption bandwidth (RL < -10 dB) was in the frequency range of 3.6-4.6 GHz. The electromagnetic loss research suggested that the superior EM absorption performances including a light weight, strong absorption, broad frequency scope, and thin thickness could be ascribed to the synergistic effect of magnetic loss from Co nanoparticles and the dielectric loss of CNTs, resulting in better impedance matching.