Abstract: Scenedesmus obliquus, a typical representative of the Phytoplankton group in freshwater ecosystems, is often used in aquatic toxicology tests and evaluation. Changes in cell density (D₆₅₀) and fluorescenceⅡparameters of Scendesmus obliquus under stresses of heavy metals ( Cu²⁺ , Zn²⁺ and Mn²⁺ ) different in concentration and different in duration ( 12 h, 24 h, 48 h, 72 h and 96 h ) were studied. The main parameters include maximal phototransformation efficiency of PSII (F_v / F_m), actual phototransformation efficiency of PSII in the light (Y), maximal relative electron transport rate (R_e,t,max), half-saturation light intensity (I_k) and light use efficiency (α) were determined. Results show that the heavy metals did not have much effect on growth of Scendesmus obliquus within 12 h and 24 h of exposure; however, Cu²⁺ and Zn²⁺ did have some significant effect inhibiting the growth of S. obliquus after 48 h of exposure. However, after 96 h of exposure, all the heavy metal treatments, except for the treatment of 100 µmol•L^-1 Mn²⁺, displayed significant effects inhibiting growth of the algae, and the treatments of Cu²⁺.did especially so. With the exposure going on, all fluorescence parameters (Fv / Fm,Y, R_e,t,max, I_k, α) decreased in value, but increased steadily in magnitude of the fall. All in all, in terms of toxicity to S. obliquus, the heavy metals displayed an order of Cu²⁺ > Zn²⁺ > Mn²⁺. The findings of the study indicate that Fv/Fm, Yield and Ik can be used as indicators to evaluate adaptability of S. obliquus to Mn²⁺ and Zn²⁺, and fluorescenceⅡ parameters can be used to evaluate adaptability of the algae to Cu^2+.