This study discussed characteristics of the combustor of the commercial circulating-fluidized-bed (CEB) boiler from a view of conceptual design. Variables relating to the material and heat balance of the boiler and to configuration of the combustor were derived. Estimation of variables was considered by measurement of operating characteristics of commercial combustors. The fluidizing pattern of the combustor corresponded to the turbulent regime and those of the dipleg and the loopseal corresponded to the bubbling or slugging regime. The combustion efficiency was achieved over 98%. Single fuel feeder was used up to 8 m(2) combustor crosssection and the additional number of feeder increased as a ratio of one per 19 m(2) combustor crosssection. Maximum width of the combustor was estimated about 6.4 m with the present feeder arrangement which located inlets on a side of the combustor crosssection. Cut diameters and overall collection efficiencies of cyclones were obtained 0.03-0.05 mm and 80-90%, respectively. The overall heat transfer coefficient on water wall of the combustor increased with the steam load and was estimated 130-160 kcal/m(2) h ℃. About 97% of total combustion would occur in the combustor and the rest of it in the recycle part.