In this paper, studies on broadband microwave absorption and electromagnetic shielding effectiveness are reported in flexible rubber composites with low filler content of nanosize conducting carbon over 8–18 GHz frequency range of electromagnetic spectrum. Rubber based composites are prepared by loading of 1–15 wt% nanosize conducting Carbon Black (CB) in silicone rubber matrix. Effect of percentage loading of nanosize CB on DC conductivity, dielectric & microwave absorption properties and electromagnetic Shielding Effectiveness (SE) of silicone rubber composites is studied. The percolation threshold is achieved at low concentration (3wt%) of CB in composites. The observed complex permittivity values revealed that composites with concentration of 5wt% CB can provide more than 90% microwave absorption (Reflection Loss > −10 dB) over 8–18 GHz at composite thickness of 1.9–2.7mm. Further, composites with concentration of 15wt% of CB shows −40 dB SE over the broad frequency range 8–18 GHz at thickness 2.8mm. The effect of composite thickness on microwave absorption properties and shielding effectiveness is also analyzed. Thus, the prepared rubber composites with suitable concentration of nanosize CB as filler may be used as microwave absorber in stealth applications as well as for EMI shielding of electronic equipments in various civilian and military areas. © 2017, Electromagnetics Academy. All rights reserved.