Butyllithium-induced anionic ring-opening polymerization of phenyl-substituted silacyclobutanes was investigated. Polymerization of 1, 1-dimethyl-3-phenylsilacyclobutane in tetrahydrofuran (THF) at −78°C proceeded in a living fashion. A linear relationship between ln[M]0/[M] and time ([M]0 is the initial concentration of the monomer and [M] is the concentration of monomer) and a linear relationship between number-average molecular weight (Mn) and monomer conversion were observed. The molecular weight of the obtained polymer was very narrow (Mw/Mn = 1.09, Mw is weight-average molecular weight). In contrast, neither 1,1-dimethyl-2-phenylsilacyclobutane nor 1-methyl-1-phenylsilacyclobutane showed a living nature under the same polymerization conditions, which were confirmed by two-step monomer addition experiments. 13C NMR and 29Si NMR spectrum of the poly(1,1-dimethyl-2-phenylsilacyclobutane) indicated that polymerization of 1,1-dimethyl-2-phenylsilacyclobutane proceeded without regioselectivity. By differential scanning calorimetry (DSC) measurements, glass transition temperatures (Tg)s were determined at −5°C for poly(1,1-dimethyl-3-phenylsilacyclobutane), 27°C for poly(1,1-dimethyl-2-phenylsilacyclobutane), and −29°C for poly(1-methyl-1-phenylsilacyclobutane).