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Flow behavior and heat transfer characteristics of air flow inside the plate pin fin heat sinks (PPFHS) are presented. The effects of pin fin shape, pin fin orientation, and ratio of distance between pin and plate fin center to pin fin size (S/Dp or S/Wp) on the flow pattern, heat transfer coefficient (HTC), pressure drop (ΔP) and thermal performance are investigated. Three types of pin fin shape, including a circular pin, square pin, and 45° square pin with pin fin sizes of 2.5, 3.0, and 3.5 mm, are used. The flow visualization used smoke to study the air flow behavior inside the PPFHS. The test runs were done at a heat flux of 14.81 kW/m2 and Reynolds number (Re) ranging between 1700 and 5200. Under the same pin fin frontal area, the HTC and ΔP of air inside the plate square pin fin heat sink (PSPFHS) was higher than that from the plate circular pin fin heat sink (PCPFHS) by an average of 12.52 and 15.05%, respectively. The decrease of the S/Dp or S/Wp from 2.25 to 1.61 caused the augmentation of the HTC and ΔP of air flow inside the PPFHS by about 11.77%–17.17% and 46.61%–50.52%, respectively. The average thermal performance factors (TPF) were 1.32, 1.44, and 1.42 for PCPFHS, PSPFHS, and the plate 45° square pin fin heat sink (P45oSPFHS), respectively. The correlations for Nusselt number (Nu) and friction factor (f) were also proposed.