Anionic ring-opening polymerization of a seven-membered cyclic carbonate, 1,3-dioxepan-2-one (1), was carried out to observe that the higher the polymerization temperature and lower the initial monomer concentration were, the lower the yield and molecular weight, and wider the molecular weight distribution of the obtained polymers were. The back-biting reaction and the formation of cyclic oligomers of 1 were observed during the polymerization of 1. The relative polymerization rate of 1 was about 35 times faster than that of six-membered carbonate, 1,3-dioxan-2-one (3). The Delta Hps of 1 and 3 estimated by MO (PM3) calculations were -9.8 and -4.4 kcal/mol, respectively. 1 could easily undergo the ring-opening polymerization based on larger ring strain.