The females of blue whales are bigger than males, and the southern blue whales are bigger than the northern blue whales. Seen from above, the kiss is wide and flat. The dorsal fin is small, about 0.4m high, and 1/4 is at the back of the body. The flippers are small, accounting for 15% of the body length. The caudal fin is 1/3 to 1/4 in length, and the trailing edge is straight. There are 55 ~ 88 puncture grooves, the longest of which can reach the navel. 270 ~ 395 whiskers per side. The back of the body is dark gray-blue, the abdomen is slightly light, and the mouth must be black.

Blue whales are different from other kinds of whales. They are short and strong, while the vertebral body of blue whales is very long and looks like it has been elongated. The head is flat and U-shaped, and the pores from the upper lip to the back have obvious ridges. There are dense baleen plates at the front end of the mouth, and about 300 baleen plates (about 1m long) hang down from the upper jaw and pass through the mouth for about half a meter. 60 ~ 90 grooves (called abdominal folds) are parallel to the body along the throat. These folds are used to discharge seawater that has been swallowed in large quantities (see the section on "predation").

The dorsal fin of the blue whale is so small that it can only be seen briefly when diving. The shape of dorsal fin varies from individual to individual; Some fins have only one protrusion just visible, while others are very eye-catching and sickle-shaped. The dorsal fin is located about three-quarters of the body length. When reaching the surface to breathe, blue whales lift their shoulders and stomata off the water to a greater extent than other large whales (such as fin whales and pomfret). This can usually be used as a useful clue to identify marine species. When breathing, if it is calm, the spectacular vertical water column (up to 12m, generally 9m) ejected by the blue whale can be seen thousands of meters away. The vital capacity of the blue whale is 5000 liters.

The fin foot of a blue whale is 3-4 meters long. The top is gray and the narrow side is white. The bottom is all white. The head and caudal fin are generally gray. But the back and sometimes flippers are usually variegated. The degree of variegated color varies from person to person, and the difference is obvious. Some may be gray all over, while others are dark blue. Gray and black are quite mixed together.

When blue whales interact with other whales, they can sprint at a speed of 50 km/h (30 mph), but usually swim at a speed of 20 km/h (12 mph). Slow down to 5km/h(3mph) when eating. Blue whales in the North Atlantic and the North Pacific lift their tail fins when diving, but most other blue whales don't.