We report the symmetry-dictated mesophase formation and phase diagram of a series of T-8-polyhedral oligomeric silsesquioxane (POSS) derivatives bearing perfluoroalkyl chains and hydroxyl alkyl groups. The phase structures and phase behaviors of these molecular Janus particles were studied by DSC, POM, FT-IR, and WAXD experiments. It was found that introducing one hydroxyl alkyl group leads to a decreased crystal melting point, and incorporating two hydroxyl alkyl groups at different POSS positions causes the formation of lamellar liquid crystal mesophases with feature sizes and transition temperatures that depend on the symmetry of these regioisomers. Interestingly, installing only one substituent with two hydroxyl groups leads to monotropic phase transitions, where the mesophase appears only upon cooling from the isotropic melt within the narrow supercooling window. Phase diagrams were systematically constructed for these compounds and understood based on the fine influence of symmetry on their possible molecular packing scheme and their distinct hydrogen bonding patterns, as evidenced by their infrared spectra at different temperatures. These results are not only fundamental to understanding the effects of regioisomerism on giant molecule self-assembly but also important for the development of hybrid materials with tailored nanostructures.