Abstract: The ultrastructure of cotyledonary mesophyll cells of Brassica napus L. were observed with the transmission electron microscope during embryo development, There were plenty of ribosomes and endoplasm reticulum and oil began to accumulate in mesophyll cells when the cmbryo was torpedo in shape. The plastid lacked fine lamella structure and a starch grain was enveloped in it. Protein deposited later then oil deposited. 30 days after pollination protein bodies appeared in the central vacuole. There were a great quantity of lipid bodies surrounding the protein bodies and were a few organelles in the cells of matured embryos. The function of plastid at the process of the embryo development was discussed.