Abstract: The cellular growth and intracellular lipid formation of Schizochytrium sp.wild-type strain ATCC20888 and mutant strain S1 were observed and analyzed by transmission electron microscope in this study. The results showed that both rates of cellular growth and oil formation of the mutant strain S1 were significantly higher than those of the wild-type strain. At the first day of growth of the mutant strain S1,single algal cell could be obviously observed. With the passage of culturing time,the cell size of S1 remained unchanged,and it was difficult to see the cysts which contained multiple algal cells. At the initial stage of cellular growth,the intracellular oil appeared as single transparent granules. Later,the oil droplets gradually grew larger and they were joined together in blocks with a dark black color. Finally,the whole cell was filled with the oil droplets. This study revealed the properties of cellular growth and lipid formation of different algal strains at microscopic level,which scientifically explained the difference of lipid yields by different algal strain. Our research provided useful guidance for further studies on the process of cellular growth as well as the mechanism of lipid formation.