In this work, pin fin and plate heat sinks were investigated in terms of natural convection and radiation heat transfer by experimental means. One rectangular base plate and eight pin fin and plate heat sinks were manufactured particularly for this study. Eight different pin fin and plate heat sinks had four different pin fin numbers and hence pin fin spacings; and two different pin fin heights. Three different orientations of 0°, 90° and 180° were tested. Ten different constant heating rates were applied to heat sinks during tests, corresponding to Rayleigh number interval between 1 × 106 and 7 × 106. Heating powers were changed between 5 and 50 W by 5 W increments by means of DC electrical power source. All cases were compared with each other. Results were evaluated by calculating heat transfer indicators from experimental measurements, dependent Nusselt and Rayleigh numbers, and by drawing their corresponding graphics. It was detected that increasing pin fin number up to a threshold value increases thermal performance. After the threshold pin fin number, thermal convection coefficient decreases significantly. Up to the favourable highest pin fin number, the reason of thermal performance enhancement is due to increasing surface area without deteriorating thermal convection coefficient significantly. It is also seen that extended surface area by increasing number of pin fins partly compensates the reduction in thermal convection coefficient up to a level. However, increasing pin fin number further degrades heat transfer performance. Results show that the highest heat transfer is achieved by 121 × 40 pin fin and plate heat sink for all three orientation angles. The lowest heat transfer performance is realized by non-pinned plate. When plate orientation is considered, the highest heat transfer is achieved with upward facing orientation which has 0° orientation angle value, and the lowest heat transfer rate is realized with downward facing heat sink which has 180° orientation angle value. Therefore, it is concluded that inline pin fin and plate heat sinks are best used with upward orientation with optimum number of pins. Experimental dataset was analysed in terms of parametrical effects and accordingly empirical correlations expressions were composed and proposed.