In this study, 90 fetuses aged from 3～10 months were collected, 48 of 90 ductus arteriosi were studied with histological method in order to observe the changes in the different fetal stages and to approach its closing mechanism. The gross morphology of the 90 ductus arteriosi and their abjacent portions were also observed. Ductus arteriosus belong to the muscular type artery. It was found that the histological structure of the ductus arteriosus after the 7th month was particulary different from that before the 7th month, the beginning of the perinatal period is the critical period of the histological changes of the ductus arteriosus, since the beginning of the perinatal period, the duct had grown rapidly, the internal elastic membrane was fragmented or split, the smooth muscular fibers of the inner layer in the tunica media remarkbly thickened and was disarranged, some oblique and spokelike smooth muscular fibers protruded into the intima through the gaps between the fragmental internal elastic membranes and formed typical intima cushions with the thickened tunica intima. From the beginning of the perinatal period, the lungs and the vessels of pulmonary circulation had been well developed, but the right and left pulmonary arteries were in the state of "the door being placed but usually closed". Blood of the pulmonary trunk flowed into the much smaller ductus arteriosus, hence the ductus arteriosus was an artery affected by local hyper-pressure, its histological changes were similar to the degeneration of the muscular artery affected by hypertension disease, but it had no lipid deposit. This local hyper-pressure may be the intrinstc cause of the histological changes of the ductus arteriosus. These histological changes were the morphological foundation of the functional closure and anatomical closure of the ductus arteriosus after birth.