Abstract: Based on the tests of the embryo viability,germination rates of whole seeds and the isolated embryos,the causes of seed dormancy of 6 species in genus of Pinus were studied by testing the biological function of the abstracts from the testa,endothelium and endosperm,and by the contents of endogenous hormones of the seed coats and endosperms of these different species.The results showed: Embryo itself of all the 6 species did not show dormancy.The inhibitors contained in the outer coats played the leading role in causing the seed dormancy,the inhibitors contained in the endosperm and endothelium were the secondary reasons to cause dormancy.ABA and its related chemicals contained in episperm and endosperm were the predominant inhibitors causing seed dormancy,and the higher ABA content and lower GA₃ content were the main causes to the seed dormancy,the ratio of ABA/GA₃ had positive relations to the extent of seed dormancy.