The Cyclization reaction of cyclodextrin glucanotransferase (CGTase, EC 2.4.1.19) from alkalophilic Bacillus sp. A2-5a, Bacillus macerans, and Bacillus stearothermophilus on amylose was reinvestigated by the use of high-performance anion exchange chromatography . When CGTase from alkalophilic Bacillus sp. A2-5a (A2-5a CGTase) was incubated with synthetic amylose, cyclic α-1, 4-glucans with degree of polymerization (DP) from 6 to more than 60 were produced in the initial stage of the reaction. The molecular mass and the cyclic structure of these glucans were determined by TOF-MS and ¹³C NMR. The larger cyclic α-1, 4-glucans produced in the initial stage of the reaction of A2-5a CGTase were subsequently converted into smaller cyclic α-1, 4-glucans and into the final major product, α-CD. CGTase from B. macerans (B, macerans CGTase) also produced larger cyclic α-1, 4-glucans, which were then converted into smaller cyclic α-1, 4-glucans and into the final major product α-CD. B. macerans CGTase, however, converted larger cyclic α-1, 4-glucans into smaller ones more slowly than A2-5a CGTase did. On the other hand, most of the α-1, 4-glucans produced by the action of B. stearothermophilus CGTase were α-, β-, and β-CD, and only a few larger cyclic α-1, 4-glucans were detected. From these results, a new model of the cyclization reaction of CGTase was proposed .