The parallel dielectric constants of several liquid crystal compounds having different core structures and polar substituents have been investigated as a function of frequency and temperature using the dilute solution by an unpolar nematic host mixture. The dielectric relaxation frequencies were found to depend roughly on the molecular length. With regard to the compounds showing a large deviation from the correlation between the relaxation frequency and the molecular length, the disagreement was explained by considering the intermolecular interaction estimated from the molecular structure.