A new four-armed tetraol, poly(1,5,7,11-tetraoxaspiro [5,5] undecane) tetraol (poly(TOSU)), was prepared by the cationic ring-opening polymerization of TOSU using BF3. OEt2 as initiator. Its structure was characterized by H-1, C-13 NMR and FTIR spectra. GPC curves showed that the polymer obtained had two fractions with higher and lower molecular weights; however, each had a relatively narrow molecular weight distribution. The molecular weights of the polycarbonate tetraols were controlled by the molar ratio of TOSU consumed to initial BHDU. The mechanism of the polymerization was discussed.