The catalytic mechanism of bacterial cellulose synthase was investigated by using a hybrid quantum mechanics and molecular mechanics (QM/MM) approach. The Michaelis complex model was built based on the X-ray crystal structure of the cellulose synthase subunits BcsA and BcsB containing a uridine diphosphate molecule and a translocating glucan. Our study identified an S(N)2-type transition structure corresponding to the nucleophilic attack of the nonreducing end O-4 on the anomeric carbon C-1, the breaking of the glycosidic bond C1O1, and the transfer of proton from the nonreducing end O-4 to the general base D343. The activation barrier found for this S(N)2-type transition state is 68 kJ/mol. The rate constant of polymerization is estimated to be similar to 8.0 s(-1) via transition state theory. A similar S(N)2-type transition structure was also identified for a second glucose molecule added to the growing polysaccharide chain, which aligned with the polymer 180 rotated compared to the initially added unit. This study provides detailed insights into how cellulose is extended by one glucose molecule at a time and how the individual glucose units align into cellobiose repeating units.