The present paper deals with experimental measurements and numerical calculations of heat transfer from unconfined rotating circular cylinder rotating in still air round its own horizontal axes. The experimental system under consideration is a rotating cylinder of 50 mm diameter placed in still air. The numerical calculations are carried out by using a finite volume method based commercial computational fluid dynamics solver FLUENT. Numerical solution was used to obtain a qualitative picture of the flow and temperature fields. The experimental measurements and numerical calculations of the heat transfer are characterized in terms of the non-dimensional parameters describing the system, i.e., the Nusselt number (Nu), rotational Reynolds (Re), and Grashof number (Gr). In this work, the experimental measurements are carried out for Reynolds number range of 1880 to 6220 and Grashof numbers range of 14285 to 714285, while the numerical calculations are carried out for Reynolds number range of 0 -100000 and Grashof numbers range of 100-1000000. In the present study, effects of rotation on the heat transfer characteristics are presented in term s of the isotherm patterns, streamlines, local and the average Nusselt numbers. The results correlated as: Nu=0.022 Re^ 0..821 This equation compares very well with the experimental and theoretical data available for air in published works.