Paeonia ostii T. Hong & J. X. Zhang (Paeoniaceae), a traditional ornamental and medicinal plant, is used as a new oil tree crop in China, but its cultivation is greatly hindered by seed dormancy. By using an alternating temperature stratification assay, the present study investigated seed germination of P. ostii. Seeds of P. ostii did not germinate under low temperature, and produced the root but not the shoot under continuous warm conditions. Seedling emergence of P. ostii was dependent on a warm (I) - cold - warm (II) sequence of temperature regimes. During warm (I) stratification, the embryo developed fully and the radicle emerged and grew; the activity of peroxidase increased in the embryo, the contents of soluble protein, IAA and ABA in the embryo decreased before radicle emerged and increased after radicle emerged, the contents of GA₃ and the GA₃: ABA ratio in the embryo increased before radicle emerged and decreased after radicle emerged,and the content of starch decreased in the endosperm.Under cold stratification, the embryo and the root developed slowly, but the GA₃: ABA ratio in embryo gradually increased. Subsequent warm (II) condition allowed the epicotyl to elongate and produce a shoot. These results indicate that seeds of P. ostii have morphophysiological dormancy. The radicle and shoot of the underdeveloped embryo have physiological dormancy (PD). Warm stratification breaks PD, allowing the embryo to grow and the radicle to emerge. Cold stratification is required to break dormancy of the epicotyl, but this treatment is not effective until after the radicle has emerged.