Natural convection is investigated experimentally in a cylindrical annulus to study the effect of angle of inclination and diameter ratio on heat transfer. Heat transfer results are given for inclination angles of 0 (horizontal), 30, 45, and 60 using annulus diameter ratios of 1.63 and 2.57. In order to verify the experimental results, the average Nusselt numbers as a function of Rayleigh number of the horizontal case are compared with those of previous experimental data for a diameter ratio of 2.6 and present predictions using the FLUENT CFD package for both diameter ratios (1.63 and 2.57) under similar experimental conditions. The predicted streamlines and isotherm contours are used to explain the heat transfer within the annulus. Empirical correlations are given for the average Nusselt number as a function of the Rayleigh number. The results show that the annulus diameter ratio and Rayleigh number influence on natural convection heat transfer in an annulus is more significant than the angle of inclination.