In this thesis, the thermal performance of radial heat sinks with a concentric ring is investigated subject to natural convection. Finite Volume Method numerical models were used to analyze the temperature and natural convection heat transfer on the surfaces of various radial heat sinks, such as heat sink with imperforated ring, heat sink with perforated ring, and chimney-based heat sink. Parametric studies are performed to investigate the effects of orientation angle with respect to gravity, various geometric parameters, and heat transfer rate on the thermal performance. In case of heat sinks with imperforated ring, a generalized correlation for predicting the Nusselt number was developed as a function of three non-dimensional parameters. The horizontally oriented radial heat sinks have better thermal performance than vertically oriented radial heat sink due to the existence of the imperforated ring preventing the natural convective upward flow. The influences of fin number and fins length on the orientation effect are significant, whereas the fin height and base height are slightly insignificant. To enhance the natural convection heat transfer, radial heat sinks surrounded by a cylinder were proposed, which was as a chimney. The results show that the chimney-based heat sink with optimal structure enhances the thermal performance by up to 20% comparing with the radial heat sink without chimney. In case of heat sinks with perforated ring, the effect of perforation number (0 - 6), perforation diameter (0 mm ? 3 mm), and perforation length (1.5 mm ? 6 mm), and orientation angle on the thermal performance were investigated. The results show that radial heat sinks with a perforated ring obviously have better thermal performance than that with imperforated ring. The geometric parameters of perforation are shown to affect the thermal performance of heat sink significantly at higher heat transfer rate, especially for upward and sideward orientations. The thermal resistance of the radial heat sink with optimal perforated ring was shown to be less than that of imperforated ring by 17%, while reducing the mass of the ring by up to 37%. The present numerical results were indicated to match the experiment results well.