Ayu has a long and narrow body with flat sides, a small head, a pointed snout, a large mouth and small eyes. The back edge is dark, and the sides and belly are white. The tail is forked, without spines, and there is a small adipose fin behind the dorsal fin. When alive, each fin is light yellow, and there is a yellow spot above the pelvic fin. The tip of the sweetfish's snout is bent downward, forming a gelsemium. There is a protrusion on each front end of the mandible, and there is an obvious depression between the two protrusions. Each of the upper and lower jaws has a row of wide, flat, fine teeth that are movable and attached to the skin. The vomer is toothless, but the jawbone and tongue have teeth. The whole body except the head is equipped with small round scales. It has an adipose fin and its body length can reach about 30 cm.