In this study, the orientation effect on the thermal performance of radial heat sinks with a circular base, concentric ring, and rectangular fins were investigated experimentally and numerically. The effects of orientation angle with respect to gravity (0 <= theta <= 180 degrees), Elenbaas number (0.5 < El <50), and non-dimensional fins spacing (0.273 <= s* <= 0.617) on Nusselt number are considered. Experimental results show that horizontally oriented radial heat sinks (theta = 0 degrees or 180) have better thermal performance than vertically oriented radial heat sink (theta = 90 degrees). The present numerical model is shown to predict the experimental results well within error of 4%, and suggests that the inferiority of vertically oriented heat sink can be attributed to the existence of the concentric ring, which can shield the natural convective upward flow. The radial heat sink in orientation 0 degrees is found to have slightly better thermal performance than that in 180 degrees when s* is relatively large, and vice versa. Based on the experimental and numerical results, a closed-form correlation with associated orientation effect factor is proposed for predicting Nusselt number, as a function of aforementioned nondimensional parameters. (C) 2015 Elsevier Ltd. All rights reserved.