An experimental study was made on heat transfer and flow characteristics behind slightly heated circular cylinders with various cylinder diameters under lower constant free-stream turbulence. The examined range of the cylinder diameter tested was from 10mm to 14mm. The obtained Nusselt number increased as either the Reynolds number or the cylinder diameter becomes larger, which definitely occurred in the case of 12mm. The laminar-to-turbulent transition region in the separated shear layer moved in the upstreamwise direction in the relatively large cylinder diameter case. It was found, therefore, that the variation of cylinder diameter has an effect on the transition phenomenon in the separated shear layer, so that the heat transfer characteristics behind circular cylinders are also influenced.