Experiments are carried out on natural convection heat transfer from square pin fin heat sinks subject to the influence of orientation. A flat plate and seven square pin fin heat sinks with various arrangements are tested under a controlled environment. Test results indicate that the downward facing orientation yields the lowest heat transfer coefficient. However, the heat transfer coefficients for upward and sideward facing orientations are of comparable magnitude. Depending on the fin structure, the performance of these two orientations shows a competitive nature. It is found that the sideward arrangement outperforms the upward one for small firming factors below 2.7, beyond which the situation is reversed. In addition, with the gradual increase in the firming factor, the performance of sideward arrangement approaches that of downward arrangement. Aside from the firming factor, the heat. sink porosity has a secondary effect on the pin fin performance. The comparison among three orientations shifts in favour of upward and sideward arrangements with raising the heat sink porosity in consequence of reducing the flow resistance. The optimal heat sink porosity is around 83% for the upward arrangement and is around 91% for the sideward arrangement. In particular, the addition of surface is comparatively more effective for the downward arrangement whereas it is less effective for the sideward arrangement. This argument is supported by showing that the augmentation factor, defined as the heat transfer of a heat sink relative to that of a flat plate, is around 1.1-2.5 for the upward arrangement, around 0.8-1.8 for the sideward arrangement, and around 1.2-3.2 for the downward arrangement. (C) 2007 Elsevier Ltd. All rights reserved.