The concentration and grain's motion in the heads of debris flows were studied experimentally. Grain motion was filmed by high-speed video camera (200fr./sec.). The observation revealed the following conditions. The front part of the head has the highest concentration and the concentration decreases with distance from the front of the head. On a fixed bed, when the bed slope is 15° and 17°, the flow is separated water and grains. A flow exists in the head, which carries grains from the center to the sides of the channel. The velocity distribution in the head is different from that of the following flow which is likely a dilatant flow. The curvature of the velocity distribution of the head is greater than the ore of the dilatant flow. The part which has the greater curvature of the velocity distribution is where the flow carrying grains from the center to the sides.