Abstract: Scenedesmus armatus was tested for its high-CO₂-tolerant mechanism. It can thrive at 60% CO₂ under control. The average growth rates were 1.228, 0.925, 0.741, 0.305, 0.042 and 0.001 g·L^-1 ·d^-1 DW at 5%,20%,40%,60%,80%,100% CO₂ conditions respectively, of which the 20% CO₂ is most applicable while 5% is most optimum. The maximum quantum efficiency of photosystemⅡ (PSⅡ) primary photochemistry, given by F_v/F_m=(F_m-F₀)/F_m, decreased distinctly in the first 24hrs indicating photoinhibition in PSⅡ, then gradually recovered to the normal levels. The microstructure and ultrastructure of cells growing under air and 20% CO₂ indicate some morphological adaptation changes: the chloroplast were not integrated as usual, in which the pyenoids are missing or seldom seen after cultured with high CO₂, besides the sizes of the cells and the number of intracellular vacuoles were larger for the cells grown in 20% CO₂ for 6 days than for those in air for the same period. These physiological and structural changes may help us to find the way in which algae adapting to the extreme-high-CO₂-stress.