In situ pyrolysis of metal-organic frameworks (MOFs) is becoming a popular technique to construct uniform carbon-based composites with excellent performance in many research fields. In this study, Co/C composites derived from a zeolitic imidazolate framework, ZIF-67, are selected as novel microwave absorbers. The obtained Co/C composites with uniform polyhedron microstructure are actually composed of amorphous carbon frameworks and highly dispersed core-shell Co@graphite nanoparticles. The pyrolysis conditions are carefully optimized, and the effects of pyrolysis temperature on carbon content, graphitization degree, magnetic property, and porous structure are also investigated. It is very interesting that these Co/C composites present different dielectric loss ability and similar magnetic loss ability, resulting in their distinguishable reflection loss characteristics. Among these candidates, the Co/C composite pyrolyzed at 800 °C (Co/C-800) shows the best microwave absorption due to its dual loss mechanisms and well matched characteristic impedance. The control experiments indicate that both high-purity Co phase and ordered microstructure are indeed helpful to improving their performances. Moreover, the effective microwave absorption frequency can be further manipulated by the polyhedron size of Co/C composites, which may provide an exciting clue for the design and fabrication of lightweight and highly effective microwave absorbers in the future. Display omitted •High-purity Co/C composites are prepared by optimizing pyrolysis of ZIF-67.•The as-prepared Co/C composites show strong microwave absorption.•Dual loss mechanism and matched impedance account for their microwave absorption.•Ordered structure and pure Co phase contribute to upgrading their performances.•The effective microwave absorption frequency can be tuned by the polyhedron size.