An experiment was conducted in a laboratory flume at the St. Anthony Falls Laboratory, University of Minnesota, to study the sorting of mixed grain sized sediments around a spur dike in a flat-bed open channel. The flow field was measured by using a micro-acoustic Doppler velocimeter (ADV). Time histories for velocities in all three spatial directions were recorded at the upstream and downstream cross sections near the dike. Results showed that two counter-rotating, secondary flow cells originated at the dike: one cell resided in the recirculation zone and grew gradually toward downstream, and the other remained in the contracted primary flow zone, decaying as it transported downstream. These measurements illustrated spatial distributions of time-averaged velocity, turbulent Reynolds stresses, and mean and turbulent kinetic energy at the neighborhood around the dike. During sediment experiments, 15cm-thick well-sorted sediment mixture was placed on channel bed. A constant flow was discharged at the upstream until a scour hole was fully developed near the dike. Sediment size distribution at the scour hole showed sediment sorting closely related to turbulent flow field around the dike. The sorting of sediment mixture primarily attributed to the turbulent Reynolds stress, which is three times as large as the mean shear stress of incoming flow. For uniformly sized material, the scouring hole will evolve to a maximum depth until the Reynolds stresses are smaller than the critical shear stresses of a single grain-sized material. In the case of non-uniform material, the surface of scour hole was armored with sediment coarser than the substrate material, so that the scour hole will not be as deep as the one formed in a mobile bed of uniform bed material.