Twenty adult cadavers were used to study the vessels of the flag flap in the dorsum of the finger with careful dissection and measurement. The results indicate: 1. The main blood supply to the dorsum of the finger comes from the cutaneous branches of the proper palmar digital artery. The dorsal digital artery is short and small, taking part in the blood supply of the dorsal digital skin of the proximal phalanges. 2. The veins communicate with each other, forming the first, the second and the third venous arch at the level of the distal phalangeal joint, the proximal phalangeal joint and the middle of the proximal phalanges. 3. The third venous arch is big. Proximally, the veins grouped to form radial and ulnar trunks, which communicate with the dorsum veins. 4. There are almost no veins in the area between the radial and ulnar venous trunk, which might be called the "vein lacking area." 5. The venous trunk, radial or ulnar, locates 4.1 +/- 0.1 mm from the midline of the finger dorsum at the middle of the proximal phalanges, and 5.2 +/- 0.2 mm at the metacarpophalangeal joint. Based on the results, we described the anatomical characteristics, the principle of design and the survival mechanism of the flag flap.