Abstract: Abstract: To explore the underlying mechanism of seed dormancy of Paris polyphylla var. yunnanensis, we study water absorption characters, germination inhibitor and endogenous mechanism during its dormancy and after-ripening process. The results showed that seed dormancy of P. polyphylla var. yunnanensis was morphysiological dormancy. The incomplete development of embryos and germination inhibitor was the main reason for seed dormancy. As the seed absorbed water easily, it would not induce seed dormancy. Endogenous hormone ABA and GA3 played an important role in seed dormancy and germination. ABA was the key factor for seeds germination, the reduction of ABA was the initiatory factor for embryo development, while the increased GA3 benefited physiological after-ri pening. Germination inhibitor may inhibit seed embryonic development; it would not relate to physiological after-ripening. In fact, the long period of low-temperature in habitat was the environmental factor for long time seed dormancy of P. polyphylla var. yunnanensis.