Based on the data from hydro-metric stations on the Yellow River and its tributaries, we have studied the grain-size characteristics of suspended sediment in relation to concentration and discharge. The middle Yellow River basin is located in a transitional zone from sub-humid to semi-arid climates, and covered by a thick loess mantle.In the neighboring desert areas in the northwest, the surface material of the Loess Plateau exhibits some remarkable areal differentiation in grain-size and forms three zones known as sandy!loess, (typical) loess and clayey loess from northwest to southeast. Controlled by these physico-geographical conditions, the grain-size of river sediment shows some particular characteristics. The plot of suspended sediment grain-size against water discharge and suspended concentration indicates some complicated variations. When the discharge or suspended concentration is low, the median size is relatively coarse. With an increase in discharge and concentration, the median size decreases to a minimum, and then increases again. During the low flow season, the relatively clear baseflow may scour the bed, so the suspended sediment is relatively coarse. Entering the rainy season, rainstorm runoff carrying fine loess material flashes to the river, making a decline in suspended sediment grain-size. However, during high-intensity rainstorms, hyperconcentrated flows often occur. Hyperconcentrated flows are a liquid-solid two-phase flow. The mixture of water and fine sediment is the liquid phase, and its unit weight is larger than clear water. The relatively coarse sediment particles suspended in the liquid phase constitute the solid phase. Hence, with the increase in concentration, more coarse particles can be set in suspension, and the median size increases again.For rivers located in different zones of surface material, some complicated relations between suspended load grainsize and mean annual suspended concentration has also been found. Double-value relations between grain-size and concentration exists, which can also be explained by the specific physico-geographical settings and the behaviors of hyperconcentrated flows in this area.