Microwave absorbers with strong absorption, broad bandwidth, thin thickness and low filler loading are highly desirable in electromagnetic absorption field. In this work, ferroferric oxide–carbon/reduced graphene oxide (Fe3O4–C/RGO) nanocomposites were fabricated by a facile solvothermal and high-temperature pyrolysis two-step strategy. Results of morphology observations demonstrated that numerous carbon frameworks with unique octahedral morphology were uniformly loaded on the crumpled surfaces of thinly flake-like RGO, and that well-designed entanglement structure consisting of octahedral carbon frameworks and crumpled RGO was observed in the as-prepared nanocomposites. Moreover, the effects of additive amounts of graphene oxide and filler loadings on the electromagnetic parameters and microwave absorption properties of as-synthesized nanocomposites were elaborately investigated. Remarkably, the obtained nanocomposites with an additive amount of graphene oxide of 2.43 wt% displayed the minimum reflection loss of ?60.5 dB at 4.8 GHz with a matching thickness of 3.6 mm and effective absorption bandwidth (reflection loss less than ?10 dB) of 5.5 GHz (from 11.2 to 16.7 GHz) with an ultrathin thickness of merely 1.5 mm. Besides, the possible microwave absorption mechanisms were proposed. This work could shed light on designing and fabricating carbon-based magnetic nanocomposites as lightweight and high-performance microwave absorbers.