Particle Image Velocimetry (PIV) was used to measure the flow field of staggered square pin-fin array in a wide rectangle channel (AR = 4). The experiment was conducted at two Reynolds number, 10000 and 20000, based on the hydraulic diameter and bulk velocity of the channel. The distribution of flow field properties was compared with that of Nu to analysis the key flow physics driving heat transfer enhancement in channel with square pin fin. The Nusselt number was achieved through temperature measurement using thermochromic liquid crystal in the same geometry setup. Results were compared with those for circular pin fin to study the effect of geometry on flow physics driving heat transfer enhancement. It was found that the wake length of square pin fin is longer than that of circular pin fin, which indicated flow around square pin fin requires longer distance to develop. Compared to circular pin fin, small scale disturbances in the shear layer of square pin fin show its contribution to local end wall heat transfer enhancement. Large motions benefit end wall heat transfer more effectively at lower Re. Small scale unsteadiness contributes more to heat transfer augment as flow develops or Reynolds number increases while large scale motions get weaker.