Dielectric and electro-optical properties of nematic liquid crystals (LCs) doped with diamond nanoparticles (DNPs) have been investigated. It is established that the effect of DNPs on the electric conductivity depends on the purity of LC; the nanoparticles adsorb ions from LCs with a substantial content of ionic impurities, but release their own ions in highly pure LCs. The DNPs also influence LC permittivity due to orientational disordering of LC and contribution to overall permittivity of the samples. In contrast to LC-Carbon Nanotubes counterparts, the LC-DNPs composites do not show memory type electro-optic switching that may be assigned to isotropic shape of DNPs and their relatively weak interaction with LC hosts.