Abstract: The formation and development of ovule and embryo sac in Populus lasiocarpa were investigated using cytochemical method for DNA, RNA, polysaccharides and proteins. The insoluble polysaccharides are at very low level in the MMC, tetrads and functional megaspore stages while RNA and protein are at higher level in all of them. The functional megaspore develops into a 8-uncleate embryo sac of polygonum type. The cytoplasmic polysaccharides are visible in 4-nucleate embryo sac. The mature embryo sac is filled up with starch grains except antipodal cells. The antipodal cells are ephemeral. The synergids have PAS positive filiform apparatus, and degenerate before fertilization. Feulgen reaction is negative in the egg nucleus but is positive in the nuclei of others in the embryo sac. The two polar nuclei move to the egg apparatus from the middle of the embryo sac before fertilization, and after contacting with the egg apparatus, they fuse with each other to form a secondary nucleus.In the early developmental stage, the ovule shows cras sinnucellate and bitegmic, Afterwards, the inner integument degenerates, so the mature ovule is unitegmic. The nuceleus at the micropylar end degenerates in 4-nucleat embryo sac stage. The embryo sac projects into the micropyle, forming a embryo sac beaker. The nuceleus at the chalazal end contains rich proteins and nucleic acids. This property is the same as that of the tapetum, and may be involved in the transfer of food materials to the embryo sac. The nutrition of embryo sac comes from starch grains which stored in the cells of ovary and placenta.