Incorporating the dielectric polarization effect induced by atomic-scale structural defects is an effective strategy to improve the electromagnetic absorption performances of materials. Herein, the defect concentration of multiwalled carbon nanotubes (MWCNTs) could be tuned by irradiation time under the 2.45 GHz microwave, depending on the localized "heat" effect. The defect density of the mostly optimized MWCNTs treated by irradiation for 4 min reached a maximum, presenting the defect distance (L-D) of 10.83 nm and the concentration (n(D)) of 2.76x10(11) cm(-2), achieving the maximum effective absorption bandwidth of 5 GHz, which is higher than original-carbon nanotubes (CNTs) (3.9 GHz). Different from the previous integration of CNTs and heterogeneous magnetic metals, the present work demonstrates a simple microwave irradiation approach for tailoring the electromagnetic absorption properties of MWCNTs by engineering the defect concentration, and this could be extended to variable carbon-related materials and diverse applications.