The catalytic properties of ${eta}-cyclodextrin$ glucanotransferase $({eta}-CGTase)$ from alkalophilic Bacillus sp. BL-12 specific for the intermolecular transglycosylation of stevioside were investigated. The molecular mass of purified ${eta}-CGTase$ by ultra-filtration and ${eta}-cyclodextrin$ polymer affinity chromatography was estimated to be 90 kDa, which is high compared to other known bacterial CGTases. The optimal pH and temperature were 9.0 and $50^{circ}C$, respectively, and thermal stability at $40^{circ}C$ was elevated 10-fold in the presence of 1% maltodextrin. The kinetic parameters of the new ${eta}-CGTase$ from alkalophilic Bacillus sp. BL-12 indicate that it is more suitable for transglycosylation than the cyclization reaction. Maltodextrin was the most suitable glycosyl donor for transglycosylation of stevioside. The transglycosylation of stevioside was carried out using 60 units of CGTase per gram of maltodextrin, 20g/L stevioside as the glycosyl acceptor, and 50g/L maltodextrin as the glycosyl donor at $40^{circ}C$ for 6h, and a conversion yield of stevioside as high as 76% was obtained.