Abstract: In order to know the mechanism of the dormancy of Sapium sebiferum seeds, we observed the changes of seed coat structure and endosperm nutrients of S. sebiferum seed during the process of maturity by using scanning electron microscope(SEM)and transmission electron microscope(TEM). The study showed that the hard and leathery seed coat composed four different parts: the cuticle, the palisade, the osteosclereids and the parenchymatous layer, arranged according to its position from the exterior to the interior. Compared to the other parts, the seed coat near radiculodium and the micropyle was thicker and the thickness increased as the seed matured. In addition, there was tallow both on the exterior and interior of the seed coat which was thought to be a barrier for exchanging water and gas with the environment. By the TEM, we found that at the early stage, the endosperm cells were characterized by the presence of amyloplasts, completeness of cell nucleus and other organelles, less black granules, and numerous vacuoles and lipid droplets. With the maturity of seed, the cell nucleus dwindled in size due to the dehydration, the starch grains transformed into sugar gradually. Maturation even led to the disappearance of vacuoles and lipid drops, and dissolution of organelles. While the black granules gathered in large quantities, indicating that the lipoprotein content increased, which would further intensify the seed dormancy.