Light emitting diode (LED) is facing thermal challenges when highly integrated in high power lighting system. Rectangular fin heat sink is widely used in LED lighting apparatuses. Experimental and numerical study of the orientation effects on the fluid flow and heat transfer of rectangular fin heat sinks under natural convection conditions are carried out. The performance evaluations for different rectangular fin heat sinks in 8 orientations are conducted. Denser fin arrays are found more sensitive to orientation. The experimental results agree well with the present prediction and previous experimental data. Correlations in a simple form No = CRam are proposed for various orientations seldom investigated before. The exponent m is found almost the same in orientation of 45 degrees, 135 degrees, 225 degrees, and 315 degrees. Furthermore, the computational results show that the mismatch between the heat transfer area and the natural convection flow and the blockage of the convection flow are the two dominant factors that deteriorate heat transfer of the rectangular fin heat sinks. (C) 2014 Elsevier Ltd. All rights reserved.