The longitudinal static second hyperpolarizabilities (γ) of one-dimensional open-shell singlet diphenalenyl radical clusters (up to trimer) are investigated using the spin-unrestricted hybrid density functional theory, UBHandHLYP, method. The longitudinal γ values per monomer for these clusters are found to significantly increase with increasing the number of monomers: the increase ratio [γ/n(n-mer)/γ(monomer)] is 2.0 (dimer) and 3.1 (trimer). The significant enhancement is predicted to be caused by the field-induced intermolecular virtual charge transfer between the both-end-phenalenyl rings, originating in the strong covalent interaction between the unpaired electrons of the cofacial phenalenyl rings.