The two subtypes of estrogen receptor (ER), ERα and ERβ, differ greatly in expression pattern and biological functions, and ERβ-selective ligands candidates to treat immune-related disorders. ERβ-selective ligands have mostly been designed based the idea of introducing a substituent that interferes sterically with the ligand's interaction with Met421 to selectively decrease the affinity for ERα (the equivalent residue in ERβ is Ile373). Therefore, we designed and synthesized a series of carboranyl phenol derivatives bearing an aliphatic substituent as candidate ERβ-selective ligands. Introduction of a longer aliphatic substituent into the carboranyl moiety enhanced the ERβ selectivity of o-carboranyl phenol derivatives 4, but not m-carboranyl bisphenol derivatives 5. Compound 4c showed 7.4-fold ERβ selectivity in ER-binding assay and exhibited moderate estrogenic activity in cell proliferation assay using MCF-7 cell line.