Cyclodextrin is a very useful compound for the food, cosmetic, pharmaceutical, and plastic industries. By comparing five different starches, corn starch was selected as substrate for cyclodextrin production. Corn starch was incubated with thermostable cyclodextrin glucanotransferase (CGTase) at a moderately high temperature without any pretreatment. The optimum temperature of used CGTase from Thermoanaerobacter sp. was 90 degrees C. This method had many advantages including no degradation of the cyclodextrins produced, efficient use of CGTase, simple removal of residual substrate by a physical method, and use of residual substrate for other products. Furthermore, this method had other advantages including no pretreatment of substrate and greater efficiency of enzyme. The conditions employed for cyclodextrin production were : substrate, corn starch; optimum reaction temperature, 65 degrees C; substrate concentration, 7.5% (w/v); enzyme, 22 U/g(-1) starch. The conversion of substrate to cyclodextrin and maltodextrin was 27.9 and 31.4%, respectively, with 40.7% substrate remaining. After removal of residual corn starch by centrifugation, the final product contained 47% of cyclodextrin; 12.68 mg cyclodextrin was produced by one unit of CGTase.