Endosperm in blotched tree peony (P. rockii cvs.) was nuclear type and of three successive development stages: free nuclear stage lasting from 4 to 29 days after anthesis (d.a.a.) , cellularization stage from 29 to 48 d.a.a. and growth and mature stage finished before 85 d.a.a. Free endosperm nuclei multiplied to 200-500 or more and the cellularization of endosperm proceeded from the micropyle to chalazal end. As endosperm developed, the cells of outer integument adjacent to endosperm degenerated to form an evident degenerated zone through which nutrients were transferred into endosperm from integument. Firstly reserved substances were mostly starch gains in the cells, and, however, were gradually reserved into the cell walls so that matured endosperm was oleaginous. Endosperm and embryo corresponded with each other in certain stages and the relationship between them was helpful for the identification of embryo development by endosperm.