Rational optimization on chemical composition and microstructure is a hot topic for magnetic carbon-based composites to overcome their challenges in microwave absorption. In this study, we demonstrate the synthesis of uniform Co/C microspheres with tunable chemical composition through a solvothermal reaction followed by high-temperature pyrolysis. It is found that this simple strategy greatly benefits from the dual functions of glucose. On one hand, glucose is the source of gluconate as the organic ligand to complex with Co ions and produce uniform Co-gluconate microspheres, and on the other hand, glucose can be converted into carbon nanoparticles and accommodated in Co-gluconate microspheres. The final Co/C microspheres display quite different electromagnetic properties as compared with pure metal Co particles. The presence of carbon component and high dispersion of Co nanoparticles induce positive reinforcements in both dielectric loss and magnetic loss. As a result, these Co/C microspheres can produce better microwave absorption performance, especially for Co/C-1.0 (the weight ratio of glucose to cobalt nitrate is 1.0), whose strong reflection loss and wideband response are also superior to those similar composites in previous reports. It is believable that this work may provide a new strategy for magnetic carbon-based composites with desirable microstructure and chemical composition. Display omitted •Highly dispersed Co/C microspheres have been successfully prepared by a MOFs-derived strategy.•Dual functions of glucose during the solvothermal process have been firstly revealed.•The chemical composition of Co/C microspheres could be easily manipulated by glucose dosage.•These Co/C nanospheres exhibit tunable electromagnetic properties microwave absorption.•Easy preparation and good performance favor the application of Co/C microspheres.