In this paper, cobalt nanoparticles were deposited on the surface of graphene by electroless plating,first sensitization and then activation using stannous chloride and palladium chloride respectively. XRD and TEM results showed that the concentrations of cobalt precursor and reductant affected the structures and yield of cobalt crystals. Electromagnetic tests results showed that the increasing cobalt content and reduction of graphene both improved the conductivity of Co-RGO. Meanwhile, Co-RGO nanocomposites had excellent absorbing properties because of magnetic loss and dielectric loss by lots of interfaces. For instance, the bandwidth of reflection below -10dB for Co-RGO*1 during 1~18GHz was about 4GHz, and the reflection for Co-RGO*2 was below -23dB from 26.5 to 40 GHz.