To get a better understanding of interactions between heavy metal lead and freshwater algae Microcysis aeruginosa Kutz. and Scenedesmus obliquus (Turp.) Kutz., the algal biomass, electrical conductivity (EC) of supernatant, concentration of superoxide anion radical (O₂^-·), activities of POD and CAT as well as the absorption of Pb²⁺ by algal were determined respectively at different concentrations of Pb²⁺, and the surface structure of two algae cells was contrasted by scanning electron microscopy. The results showed as follows: (1) 3 mg/L of Pb²⁺ accelerated the growth of M. aeruginosa, and the opposite effects were observed when exposed to more than 9 mg/L of Pb²⁺; however, the growth of S. obliquus was obviously inhibited when the concentrations of Pb²⁺ ranged from 3 to 12 mg/L. The effects of Pb²⁺ on the two kinds of freshwater algae indicated that S. obliquus was more sensitive to the Pb²⁺ pressure. (2) The membrane permeability of the two algae cells was changed under the stress of Pb²⁺. The photos of scanning electron microscopy showed that the superficial flocs of the two algae cells increased with the increasing of Pb²⁺, and numerous cells deformed or ruptured evidently, especially for S. obliquus. Meanwhile, the increases of O^-₂ contents were exhibited and the activities of POD and CAT in the two algae were advanced with increasing of Pb²⁺ concentration at the early stage, which indicated that the main mechanisms of Pb²⁺ acting on two algae involve the increasing production of superoxide anion radical and the accompanying physiological and biochemical changes of the algae. (3) Pb²⁺ can be absorbed by the two algae, and moreover S. obliquus had higher capacity of Pb²⁺ absorption than M. aeruginosa did at the unit cells. From the above points, compared with the M. aeruginosa, S. obliquus can be used not only as a biological indicator for monitoring heavy metal pollution levels, but also as a biological material to deal with water Pb²⁺ contamination due to its ability of Pb²⁺ absorption.