The hybrid network of Si3N4 whiskers and conducting carbon fiber has great potential for microwave absoprtion applications. The high electrical conductivity of the carbon fiber helps to transform the microwave transparent Si3N4 into microwave absorbing materials. Herein, the microwave absorption performance of 5–20 vol % of carbon fiber reinforced reaction bonded Si3N4 (Cf-RBSN) composites have been discussed in detail. The Cf reinforcement tuned the X-band dielectric properties of the RBSN composites. The 5 vol % Cf-RBSN composite exhibit a minimum reflection loss (RLmin) of −36.16 dB (99.998% microwave absorption) at 11.89 GHz and a high specific reflection loss of 920 dB. g−1 for 5.9 mm thickness, while 20 vol % Cf-RBSN composites resulted in RLmin of −22.86 dB at 11.56 GHz with a low thickness of 1.5 mm. Thus, the superior microwave absorption performance of the prepared lightweight composites results from the multiple interfacial polarization, dipole polarization, and conduction loss due to the 3D network of interconnected Si3N4 whiskers and Cf. © 2021 Elsevier Ltd and Techna Group S.r.l.