Conductive composites were prepared by filling high density polyethylene (HDPE) with various amounts of multi-walled carbon nanotubes (MWNTs) and were characterized by using a terahertz time-domain spectroscopy (THz-TDS) setup assisted with conductivity measurements. The direct current (dc) conductivity measurement indicated that the percolation threshold was about 5%in volume fraction. The frequence and filler concentration dependences of alternating current (ac) conductivity were investigated and the results could be understood with the polarization effects between clusters, anomalous diffusion within the clusters as well as the percolating structures. With increasing the MWNT particles, the absorption coefficient and refractive index all increased at THz band due to the increase of the charge carrier density. The experimental data were analyzed with Cole-Cole formula of dipole relaxation under the assumption that conductive particles dispersed in the matrix behaved like dipoles and contributed mainly to the dielectric loss in the THz frequency range.