Ortho-phenylene derivatives (OPs) have tightly packed n-phenylenes with pi-pi stacking interactions. Although OPs have high activation energy for helical inversion, these helices undergo rapid helical inversion in acetonitrile solution. The solvent effect on helical inversion was investigated by using density functional theory calculations with the polarizable continuum model. We found that the dihedral angles become small, i.e., the twist becomes strong, with an increasing dielectric constant, and the OPs may take the symmetric helical inversion pathway in solution.