Composites containing polyacrylonitrile (PAN) and different mass contents of multiwall carbon nanotubes (MWCNTs) were prepared and structurally investigated. X-ray Diffraction of pristine PAN reveals the presence of crystalline and amorphous phases which change their ratio under thermal annealing and addition of MWCNT. For as prepared samples, thermal analysis reveals two glass transition temperatures, which support the hypothesis that unoriented PAN is a two-phase material. Infrared spectrum of as prepared PAN suggests that the polymer is not stabilized. Dielectric investigations of PAN/MWCNT composites show that permittivity has a strong increase as the MWCNT mass content increases, while the dielectric losses are comparable in all samples. These results suggest that PAN/MWCNT composites could find important applications in electronics.