The cationic polymerization behavior of 2,3-dihydrofuran (DHF) was investigated. Polymerization of DHF was carried out in the presence of boron trifluoride (BA) or ethylaluminum dichloride (EADC). The glass transition temperature (T-g) of the polymer depended upon the polymerization conditions. The T-g of the polymer obtained in the polymerization with BF3 at -78 degrees C was 150 degrees C. On the other hand, the T-g of the polymer obtained in the polymerization above -60 degrees C was ca. 130 degrees C. In the polymerization with EADC, addition of ethyl acetate increased the molecular weight of the polymer compared to that of polymerization in toluene only. In the C-13-NMR spectra of the polymer, splits of signals of main chain carbon atoms which corresponded to erythro and three isomers were observed. A linear relationship between the ratio of isomers and T-g was confirmed. The all-trans conformer of the threo-diisotactic isomer was concluded to be the most stable one from MM2’ calculation. C-13-NMR and molecular mechanics studies suggested that the difference in T-g of the polymers was derived from the difference in tacticity of the polymers.