This paper dealt with the ovule ontogenesis and development, the megasporogenesis and the embryo sac development in Paeonia rockii cvs. The ovule, originated from the tri-zonate primordium, was anatropous, bitegmic and crassinucellate. And the nucellus consisted of different cytohistological zones and differentiated by a particular way in which the derivative cells from both parietal and sporogeneous cells were contributed to the nucellar tissue. During the megasporogenesis and before the 4-nuclear embryo sac, the megasporocytes successively entered meiosis or commenced to degenerate. Generally a linear tetrad formed after meiosis and the chalazal megaspore was functional and developed into embryo sac by Polygonum type. Multicelled archesporium and multiple megasporocytes in an ovule resulted in 1-3 embryo sacs, which developed to mature within several days after anthesis. Belated mature of embryo sacs supplies a device to guarantee the cross-pollination. The characteristics about ovule and embryo sac of peonies were more primitive and imply the primitive of Paeoniaceae in phylogeny