In this study, horizontally oriented radial plate-fin heat sinks in natural convection are optimized analytically and experimentally. The thermal resistance is selected as an objective function under the constraint of a given base-to-ambient temperature difference. For thermal optimization, a new correlation of the heat transfer coefficient for the radial plate-fin heat sinks is developed using the asymptotic method and validated experimentally. While the existing correlation has a limited applicable range, the newly developed correlation covers complete ranges of the three design parameters for a fin geometry: fin thickness (t), fin length (L), and number of fins (n(fin)). Using this new correlation, the thermal performance of a radial plate-fin heat sink is optimized with respect to these three design parameters. Finally, optimum fin geometries for applications with various sizes are obtained, since the newly suggested correlation is applicable to various physical sizes which are characterized by the base diameter (D) and the fin height (H).