Partial molar volumes of polycyclic aromatics (benzene, naphthalene, anthracene, phenanthrene, chrysene, diphenyl, o-, m-, p-terphenyl) at infinite dilution were measured. The additivity rule with respect to the number of aromatic rings holds very accurately for the set of solute molecules of benzene, naphthalene and phenanthrene, as well as the set of benzene, diphenyl, and p-terphenyl. The linearity of the set of data of benzene, naphthalene and anthracene is slightly worse. The fact that the additivity rule holds including monomer for these series of polycyclic aromatics makes a sharp contrast with the case of n-alkane, where the partial molar volume of methane is somewhat larger than the value expected from the additivity rule. In order to explore the origin of this difference, theoretical calculations were made for model chain molecules. The calculated result suggests that this difference between polycyclic aromatics and iz-alkane is not caused by the size difference between monomer and solvent molecule. The partial molar volume of isomers of terphenyl becomes smaller as the molecule takes more stretched form, which is quite opposite to many other compounds. This peculiar behavior is referred to the increase of dihedral angle between two aromatic rings along with the decrease of bond angle.