The catalyst layer was introduced by a novel slurry impregnation process. In this process, phenolic resin, Ni particles and SiO2 particles are acted as adhesives, catalysts and dispersants, respectively. CNFs which have a high length-diameter ratio were in situ grown on the surface of carbon fabrics via the CVD method. The microstructure, electrical conductivity, skin depth and electromagnetic interference shielding effectiveness (EMI SE) of CNFs/CF under different reaction times were investigated. The optimal process of CNFs/CF was obtained at 800 ∘ C for reacting 15 min. In this condition, CNFs have a good graphitization degree and the diameter is around 25 nm. Since the CNFs/CF composite possesses numerous channels, the electrical conductivity increases and the skin depth decreases. The electromagnetic (EM) waves could be trapped and attenuated, yielding outstanding EM shielding effectiveness. The total shielding effectiveness (SE T) values were significantly under the ultrathin thickness. Consequently, the specific SE T values of CNFs/CF reached 172.9 dB/cm, which means that the EM shielding effectiveness is excellent. The slurry impregnation and CVD method were combined to successfully realize the fabrication of CNFs/CF composites. When the temperature reached 800∘C, the best morphology of CNFs was obtained. The total shielding effectiveness (SET) values were significantly under the ultrathin thickness (0.22 mm). The specific SET values of CNFs/CF reached 172.9 dB/cm, which means that the EM shielding effectiveness is excellent. [ABSTRACT FROM AUTHOR]