Carbon black (CB) was dispersed into high density polyethylene (HDPE), which was transparent to terahertz wave, by melt-mixing and compressing method. The dielectric properties of the composites in terahertz region were then investigated. Reversed effective medium approach (REMA) was firstly applied to the analysis of the composites. The real part and imaginary part of dielectric constants, refractive index and absorption coefficient of the pure CB particles were extracted with REMA from that of the composites with fixed concentration and an alterable parameter of depolarization factor. The results indicate that the calculated dielectric constants, absorption coefficient and refractive index of CB from different concentrations coincide well with each other. Combined with the direct current conductivity test, the depolarization factor was found to be correlated to the CB concentration and its distributing state in the composites. It has an apparent decrease at the percolation threshold of CB/HDPE composites. The extracted results were then analyzed with the dipole relaxation model. The relaxation time, relaxation strength and dc conductivity of the pure CB particles were obtained.