The electromagnetic shielding effectiveness (SE) of cement based composites filled with carbon black and carbon fiber were discussed in this paper. Results show the SE of cement composites will get obvious improvement as the carbon black mass fraction increases beyond 6%. Carbon fiber is a much more effective additive than carbon back. The shielding effectiveness were gradually improved with the increase of content of carbon fiber, the maximum attained 21 dB at 1.5GHz. Adding carbon black and carbon fiber, SE of the cement based materials get more upward gradient, the maximum SE reached 27 dB in tested frequency range.