The flow past the rotating circular cylinder and the effect of buoyancy on heat transfer char-acteristics are studied numerically for the Reynolds number of 20 and 40 and the Prandtl number of 0.7. The lift and drag coefficients, Strouhal number, and local Nusselt number on the cylinder are studied under the sway of combined buoyancy (at the Richardson num-ber varies from 0 to 2) and different rotational directions. Although the interaction between buoyancy and rotation is a puzzling heat transfer problem, the direction of rotation is found to have significant effects on the flow patterns and heat transfer rate. The main innovation of the present work is to determine the extreme points of Nusselt numbers when different conditions are applied. For a positive rotation, the maximum local Nusselt number is at θ=225o, and the minimum local Nusselt number is at θ=100o. In contrast, for a negative rotation, the maxi-mum and minimum local Nusselt numbers are at θ=140o and θ=270o, respectively. Applying Taguchi method, it is found that average Nusselt number is more dependent on Reynolds number than other factors. Additionally, it can be concluded that the direction of rotation can be used as a powerful tool to adjust the heat transfer rate and the required value of drag and lift. Consequently, without applying different rotation speeds, it would be difficult to stabilize the flow, and with the aid of Taguchi method, it is determined that rotation is deciding factor in stabilizing flow patterns. The results are in good agreement with the experimental results.