Flow statistic in the mid-plane of a rectangular channel with diamond-s pin fins was obtained by means of particle imaging velocimetry at Re = 10,000. Large-scale and small-scale fluctuations were separated using proper orthogonal decomposition. The flow characteristics were compared to the Nusselt number distribution on the endwall acquired by thermochromic liquid crystal to reveal the flow mechanism driving heat transfer enhancement. Results indicate that local vorticity plays an important role in strengthening Nu on both sides of leading point (Zone 1). Downstream of the two sharp edges on both sides (Zone 2), small size disturbances from shear layer eddies drive local heat transfer. The flow characteristics and heat transfer distribution downstream of the first row (Zone 3) present alternated feature along Y direction due to the interaction between shear layers of neighboring pin fins. Lateral velocity fluctuation induced by large vortex shedding drives the heat transfer augmentation in Zone 3 where there is violent large vortex shedding. Meanwhile, small size disturbances of the shear layer drive local heat transfer enhancement in Zone 3 downstream of pin fins where large vortex shedding is suppressed. For the second and third rows, there is no difference in the flow characteristics downstream of neighboring pin fins. Small-size fluctuations distributed uniformly downstream of large vortex shedding (Zone 4) resulting in a uniformly distributed Nu.