The nano-sized carbon black (NCB) with particle sizes of 15, 25, 55 and 65 nm are added in epoxy matrix in order to investigate the impact of particle sizes on the microwave absorption properties of the nano composites. The NCB powders were characterized by X-ray diffraction and Scanning electron microscopy (SEM) for identification of impurity phases. The complex permittivity spectra were recorded for the composites in the frequency range of 8.2-12.4 GHz. The results revealed that the particle sizes had substantial impact on the electromagnetic properties. On decreasing the particle size, the complex permittivity increases first up to certain extent then decreases. Interestingly, the particle size dependent tangent loss of NCB/epoxy composites shows that 25 nm size is most suited for high loss characteristics, whereas, losses decreases with size < 25 nm, which is attributed to high surface area of the particles. In addition, NCB/epoxy composite with 25 nm particle size illustrated substantial EM absorption efficiency at a frequency of 9.5 GHz, with RL reaching −13.2 dB and reflection loss below −10 dB up to 2.5 GHz with thickness of 2.0 mm. The effect of particle sizes on the electromagnetic properties was also discussed. Copyright © 2018 VBRI Press.