Abstract: The planar anisotropic carbonyl iron (PACI) was firstly fabricated by mechanical ball milling. Then the carbon nanotubes(CNTs)/PACI composite was prepared by solution blending method. The complex permittivity and complex permeability of the samples between 2-18 GHz were measured through coaxial method. The effects of the CNTs doping content on the electromagnetic properties of CNTs/PACI composites were investigated. The results show that the CNTs/PACI composites have higher complex permittivity and attenuation constant compared with PACI.The complex permittivity and attenuation constant of the composites increase gradually with the increasing mass fraction of CNTs (W_CNTs), while the microwave impedance decreases gradually. Doping CNTs can effectively improve the microwave absorption properties of CNTs/PACI composites. Through adjusting the thickness (d) and the CNTs doping content, the microwave absorption properties of the composites can be effectively controlled. When d=1.2 mm and W_CNTs=2wt%, or d=1.6 mm and W_CNTs=0.5wt%, the reflection loss (R_L) of CNTs/PACI composite in Ku band (12-18 GHz) is below -10 dB. When d=2.0 mm, W_CNTs=0.5wt% and 1wt%, the bandwidth corresponding to the R_L of the composites below -10 dB is 5.28 GHz (8.24-13.52 GHz) and 5.04 GHz (7.52-12.56 GHz) respectively, which covering the entire X band (8-12 GHz).