Microwave absorbing composites with epoxy-cyanate ester (EP-CE) as matrix and graphene nanosheets (GNSs) as absorbers were prepared, and their electromagnetic and microwave absorbing properties were investigated in the frequency range of 2.6–12.4 GHz. The microstructures of the composites showed a uniform dispersion of the GNSs in the matrix. The complex permittivity of the composites increased with increasing filler content. As absorbing materials, the GNSs/EP-CE composites possess excellent absorption properties. And the composites with 3 wt.% of GNSs exhibited high values of reflection loss (>10 dB) over a wide frequency range 5.8–6.6 GHz and maximum loss is 15.7 dB at 5.8 GHz at a thickness of 3 mm. The minimum reflection loss was found to move toward to the low frequency region (from 10.2 to 4.5 GHz) with increasing composites thickness. Especially, a minimum reflection loss value of −21.4 dB was obtained at 4.5 GHz for the composite thickness of 4 mm. Moreover, the thermal properties of the composites were also investigated in detail. It is observed that GNSs reinforced EP-CE composites demonstrated better thermal stability than that of EP-CE matrix, which make them suitable for use in aerospace applications and radar absorbing material (RAM).