Abstract: The periodicity of vascular cambial activity in Populus deltoides could be divided into four stages: dormant, renewed active, active and transformed stages. In the dormant stage, the vascular cambium was composed of three layer cells. The cell had a thicker wall than the usual cambial cell, and also had a lot of small vesicles containing some substances that had higher electronic density. The dissociative ribsomes were abundant in the cytoplasm. Pellet-shaped lipid grains spread among the vesicles. Most of smooth endoplasm reticulum (SER) looked like short pieces in cell, but the SER was reticulated near the cell wall. The cristae of mitochondria were not clear either. Usually, the Golgi body that was formed by 3～5 cisternaes was rare. There was no Golgi vesicle around the inactive Golgi body. In the renewed active stage, the cambial zone cell wall became much thinner, and the electronic density in cytoplasm was much lower because the substances involved in vacuole were disaggregated. In the active stage, the vascular cambial zone was composed of 7～8 layer cells. The cell wall was the thinnest one during the cambial activity period. At the same time, the cell was filled with vacuole mostly, and the cytoplasm and the nucleus were extruded beside the cell wall, or was overhang somewhere by cytoplasmic cable. The structure of organelle such as mitochondria, Golgi body, endoplasm reticulum in the cell was complete. There were plenty of plasmalemma invaginations in the cell, which was related to cell wall substances accumulation. In the transformed stage, the mitosis of vascular cambial cell ceased, and the cell wall became thicken again. The volume of vacuole decreased, but the amount of vacuoles increased. Synchronously, starch, lipid and protein began to accumulate in the cell. During the periodicity of vascular cambial activity, the ultrastructural character of dormant stage was opposite to that of active stage completely. However, the ultrastructural character of the renewed active looked like that of the transformed stage, it could be a reversible process.