A creative fabrication strategy is proposed for synthesizing two kinds of hierarchical graphene-based and MWCNT-based full-carbon nanostructures (FCNS), both of which are excellent microwave absorption (MA) fillers. Through using precursor graphite and MWCNTs, the FCNSs are prepared by simple ultrasonic exfoliation/dispersion and exothermal erosion methods. The hierarchical carbon nanostructures (carbon nanoparticles, carbon nanofilms and carbon hollow hemispheres) and abundant interfaces in FCNSs generate strong dielectric loss. Consequently, the FCNSs present ideal microwave impedance match, attenuation characteristics and excellent MA properties. Results show that the MWCNT-based FCNS and graphene-based FCNS composites achieve extremely strong absorption with minimum microwave reflection loss (RL) values of - 58 dB with a thickness of 2.6 mm and - 54 dB with a thickness of only 1.2 mm, respectively. And the effective absorption bandwidth (RL < -10 dB) can be reached to 3.44 GHz at 1.4 mm and 4.3 GHz at 1.1 mm, respectively. Moreover, we also elaborate on the fundamental MA mechanism and correlation between the nanostructures and MA properties. The ultrathin MAM can be used as predominant candidate with strong MA properties and wide bandwidth.