An experimental study is conducted on natural convection heat transfer from square pin fin heat sinks subject to the influence of orientation. A total of six pin fin heat sinks with various arrangements are tested under controlled environment. Test results show that the upward facing orientation yields the highest heat transfer coefficient, followed by the sideward facing and the downward facing ones. The heat transfer coefficients for upward facing are about 0% ∼ 5% greater than those for sideward facing and are 0% ∼ 20% greater than those for downward facing. Besides, the effect of orientation becomes less pronounced as the population density is gradually increased. It is also found that the heat transfer coefficient reveals a maximum at a fin height nearby 2 mm or at a fin density of 1 pin/cm2. The effect of fin height and fin density can be termed as a composite parameter, namely the finning factor which represents the ratio of total surface to the base surface. The heat transfer coefficient initially increases with the finning factor, attains a maximum around 1.6-1.7, and then decreases. Although the heat sinks may not necessarily surpass the un-finned base plate in the heat transfer coefficient, the heat sinks still yield smaller convection resistances than the un-finned base plate.