The cyclopolymerization of 1,2:5,6-dianhydro-D-mannitol (1) was carried out using (BF3OEt2)-O-. and t-BuOK. Although the anionic polymerization tended to form gels, the cationic polymerization proceeded through the proton-transfer reaction mechanism to produce hyperbranched carbohydrate polymers (2) mainly consisting of 2,5-anhydro-D-glucitol units. The weight-average molecular weight (M-w,(SLS)) values of 2 measured by static light scattering (SLS) varied in the range of 2.08 x 10(5)-26.9 x 10(5), which were significantly higher than the weight-average molecular weight (M-w,M-SEC) values by size exclusion chromatography (SEC). The degree of branching (DB), estimated by the C-13 NMR measurements, was ca. 0.44-0.46. The alpha value of the Mark-Houwink equation, which was determined by the viscosity measurements, was ca. 0.3. The hyperbranched polymers 2 were nanoscale particle with the radii of gyration (R-g) of 67.4-132.0 nm.