An experimental study of natural convection in a cylindrical enclosed annulus filled with air has been carried out to investigate the effect of diameter ratio and the angle of inclination on the rate of heat exchanged across it. In different case studies the inner cylinder surface is heated up at a uniform heat flux, surrounding atmospheric dir convectively cools the outer cylinder The annulus is long with respect to the gap width (L/8>>1). The experiments are performed in the horizontal case for modified Rayleigh number (R α. S)α ranging from 5 to 105 and diameter ratio of 1.15 /DoDi2.3. Heat transfer results are also given for the annulus of Do/Di=2.88 all different inclination angles. For the horizontal case, the average heat transfer coefficient is found to be strongly dependent on flow regime. Over the lower range R α . S 1.3x103, the effect of both Rayleigh number and the diameter ratio is diminished. For the higher range 1.3x103≤ R α . S≤ 105, the average heat transfer coefficient is strongly affected by Rayleigh number as well as the diameter ratio. The results also show that the increase of inclination angle of the annulus axis causes an increase in the rate of convective heat transfer, while the effect of Rayleigh number diminishes for 0>60" Empirical correlations are given for the average Nusselt number as a function of the modified Rayleigh number and comparisons are made with the available literature.