Abstract: The reasons for the dormancy of Osmanthus americanus seed were discussed in this paper. The experiments e. g. seed morphology anatomy, electron microscopic observation on structure of seed coat, the determination for water permeability of seeds, and biological identification for inhibitor in structures of seed were carried out. The results indicated that the lignified seed shell had a negative impact on water permeability of seeds, but it was not the main reason for seed dormancy. Without any phenomenon of dormancy, the embryo in vitro began to germinate on the 7th day after light treatment. Because of encapsulation of endosperm, the necessary water and oxygen could not be available to embryo for its germination. This could be the main reason of seed dormancy.