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Nowadays, developing novel microwave absorbers with thin matching thickness, strong absorption intensity, broad absorption bandwidth and low filler loading is of great importance for solving the increasingly serious problem of electromagnetic pollution. Herein, nitrogen-doped cobalt oxide/cobalt/carbon (CoO/Co/C) nanocomposites were fabricated by high-temperature pyrolysis of heterobimetallic zeolitic imidazolate frameworks (Co/Zn-ZIFs). Results demonstrated that the micromorphology and magnetic properties of as-prepared nanocomposites could be regulated by changing the molar ratios of Co to Zn in the ZIFs precursors. Moreover, the effects of molar ratios of Co to Zn and filler loadings on the microwave absorption properties of obtained nanocomposites were systematically investigated in the frequency range of 2–18 GHz. Remarkably, the asprepared nanocomposites with a molar ratio of Co to Zn of 1:1 exhibited the best microwave absorption properties. The optimal minimum reflection loss reached  66.7 dB at 7.2 GHz with a thickness of 3.3 mm and effective absorption bandwidth (EAB) achieved 5.1 GHz (12.6–17.7 GHz) covering most of Ku-band for an ultrathin thickness of 1.8 mm and a low filler loading of 25 wt%. Furthermore, the EAB could reach 14.16 GHz (88.5% of 2–18 GHz) by facilely modulating the thicknesses from 1.5 to 5 mm, which spanned the whole C, X and Ku bands. In addition, the underlying microwave absorption mechanisms were carefully investigated and further proposed. Therefore, our results could be helpful for designing and fabricating the magnetic nanoparticles/carbon nanocomposites derived from ZIFs as lightweight and high-efficient microwave absorbers.