OBJECTIVE To investigate the mechanism of necrosis of avulsion injured skin flap. METHODS Six swine were used. An avulsed skin flap and traditional island flap were made on each hind leg of the swine respectively, and the latter was used as control. Then the microvascular casting of each flap was made routinely immediately following the injury and 72 hours later, and the vascular casting were observed under electron microscopic scanning. RESULTS There were three characteristic changes in the avulsed skin flap: tearing of blood vessels, formation of complete thrombosis and incomplete thrombosis. If these changes were excessive in the avulsed flap, then the necrosis was the result, otherwise, the flap survived. CONCLUSION The necrosis of avulsed flap was related to the type and extent of the damage of microvascular endothelial structure. So the degree of endothelial damage was the keypoint in the viability of avulsed flap.