Metal cobalt is one of the most promising candidates for high-performance microwave absorbers due to its compatible dielectric loss and magnetic loss abilities. Rational design on the microstructure of metal cobalt became a popular way to upgrade itsmicrowave absorption performance in the past decade, while much less attention has been paid to the electromagnetic functions derived from its different crystal structures. Herein, we report the microwave absorption of porous cobalt assemblies with varied composition of close-packed hexagonal (hcp) and face-centered cubic (fcc) phases. Electromagnetic analysis reveals that the change of phase composition can significantly impact the complex permittivity and complex permeability ofmetal cobalt, where hcp-cobalt favors high complex permittivity and fcccobalt produces high complex permeability. The optimum phase composition in these porous cobalt assemblies will promise well-matched characteristic impedance and good performance in strong reflection loss (-41.0 dB at 9.4 GHz) and wide response bandwidth (4.0-17.4 GHz over -10.0 dB). The enhanced microwave absorption is superior tomany cobalt absorbers ever reported. It is believed that these resultswill provide a new pathway to the design and preparation of highly effective metal cobalt andcobalt-based composites asnovelmicrowaveabsorbers inthe future.